Part 17 (1/2)
The plant grows abundantly in waste places, and is in flower nearly the year round, so found in flower in midwinter, after a week or two of eather It is, however, in best condition for study in the spring and early sunized by the heart-shaped pods and sins to flohen very s plant, half a e tap-root (Fig 93, _A_, _r_) is seen, continuous with theplant continues here as the ymnosperms, but not with the monocotyledons
From this tap-root other s a co to the fornified (Fig 93, _M_), shows a round, opaque, white, central area (_x_), the wood, surrounded by a(_ph_), the phloeround tissue and epidermis
The lower leaves are crowded into a rosette, and are larger than those higher up, fro a stalk (petiole), while the upper leaves are sessile The outline of the leaves varies much in different plants and in different parts of the sa sometimes almost entire, sometimes divided into lobes alradations are found The larger leaves are traversed by a strong ly on the lower side of the leaf, and from this the smaller veins branch
The upper leaves have frequently two s from the base of the leaf, and nearly parallel with the hened with hairs, sonified, look like little white stars
Magnifying slightly a thin cross-section of the steround tissue (pith), whose cells are large enough to be seen even when very slightly enlarged Surrounding this is a ring of fibro-vascular bundles (_L_, _fb_), appearing white and opaque, and connected by aof fibro-vascular bundles is the green ground tissue and epider pine steement was also noticed in the steiven off fro at the point where the leaves join the stem (axils of the leaves), and these may in turn branch All the branches terated inflorescence, and the separate flowers are attached to the main axis of the inflorescence by short stalks This form of inflorescence is known technically as a ”raceme” Each flower is really a short branch from which the floral leaves arise in precisely the sae leaves do from the ordinary branches There are five sets of floral leaves: I four outer perigone leaves (sepals) (_F_), sreen, pointed leaves traversed by three sier, white, inner perigone leaves (petals) (_G_), broad and slightly notched at the end, and tapering to the point of attachment The petals collectively are known as the ”corolla” The veins of the petals fork once; III and IV two sets of sta two short, and the inner, four longer ones arranged in pairs Each stamen has a slender filament (_H_, _f_) and a two-lobed anther (_an_) The innermost set consists of two carpels united into a cohtly flattened so as to be oval in section, and divided into two chambers The style is very short and tipped by a round, flattened stig ties of flowers and fruit may often be found in the same inflorescence
The flowers are probably quite independent of insect aid in pollination, as the stamens are so placed as to alma This fact, probably, accounts for the inconspicuous character of the flowers
After fertilization is effected, and the outer floral leaves fall off, the ovary rapidly enlarges, and becoles to the partition When ripe, each half falls away, leaving the seeds attached by delicate stalks (funiculi, sing
funiculus) to the edges of the membranous partition The seeds are s, yellon shell, and with a little dent at the end where the stalk is attached Carefully dividing the seed lengthwise, or crushi+ng it in water so as to remove the embryo, we find it occupies the whole cavity of the seed, the young stalk (_st_) being bent down against the back of one of the cotyledons (_f_)
[Illustration: FIG 94--_A_, cross-section of the ste a fibro-vascular bundle, 150 _ep_ epiderround tissue _sh_ bundle sheath _ph_ phloe root seen in optical section, 150 _r_, root cap _d_, young epider fibro-vascular bundle _C_ cross section of a small root, 150 _fb_ fibro-vascular bundle _D_, epidermis from the lower side of the leaf, 150 _E_, a star-shaped hair from the surface of the leaf, 150 _F_, cross-section of a leaf, 150 _ep_ epiderround tissue _fb_ section of a vein]
A microscopic examination of a cross-section of the older root shows that the central portion islines of thick-walled cells (fibres) interspersed with lines of larger, round openings (vessels) There is a ring of s into the phloeular cells, with pretty thick, but soft walls The ground tissue is coe, loose cells, which in the older roots are often ruptured and partly dried up The epideruishable in the older roots To understand the early structure of the roots, the smallest rootlets obtainable should be selected The smallest are so transparent that the tips may be mounted whole in water, and will show very satisfactorily the arrange tissues The tissues do not here arise frole, apical cell, as we found in the pteridophytes, but fro 94, _B_) The end of the root, as in the fern, is covered with a root cap (_r_) co point The rest of the root shows the same division of the tissues into the pri fibro-vascular cylinder (pleroround tissue (periblem) (_pb_) The structure of the older portions of such a root is not very easy to study, owing to difficulty ina very sharp razor, and holding perfectly straight between pieces of pith, however, satisfactory sections can be made The cells contain so much starch as to make them almost opaque, and potash should be used to clear them The fibro-vascular bundle is of the radial type, there being twothe two phloem masses (_ph_), some of whose cells are rather thicker walled than the others The bundle sheath is not so plain here as in the fern The ground tissue is coe cells with thickish, soft walls, that contain much starch The epiderer roots the early forerowth, soon obliterate all traces of the pri a thin cross-section of the ste the section a single row of epider 94, _A_, _ep_) whose walls, especially the outer ones, are strongly thickened Within these are several rows of thin-walled ground-tissue cells containing numerous small, round chloroplasts
The innerer and have but little chlorophyll This row of cells for of fibro-vascular bundles very much as is the case in the horse-tail The separate bundles are nearly triangular in outline, the point turned inward, and are connected with each other by masses of fibrous tissue (_f_), whose thickened walls have a peculiar, silvery lustre Just inside of the bundle sheath there is a row of si the outer limit of the phloem (_ph_) The rest of the phloem is composed of very small cells The xylem is coe vessels (_tr_) The central ground tissue (pith) has large, thin-walled cells with numerous intercellular spaces, as in the stem of _Erythronium_ Some of these cells contain a few scattered chloroplasts in the very thin, protoplas their walls, but the cells are alitudinal section shows that the epiderround tissue less so, and in both the partition walls are straight In the fibrous cells, both of the fibro-vascular bundle and those lying between, the end walls are strongly oblique The tracheary tissue of the xyleer ones with thickened rings or with pits in the walls The small, spirally-marked vessels are nearest the centre, and are the first to be for bundle
The epiderular cells avy outlines like those of the ferns Breathing pores, of the same type as those in the ferns and monocotyledons, are found on both surfaces, but more abundant and more perfectly developed on the lower surface of the leaf Owing to their small size they are not specially favorable for study The epiderly covered with unicellular hairs, soularly star-shaped The walls of these cells are very thick, and have little protuberances upon the outer surface (Fig 93, _E_)
Cross-sections of the leafthe leaf carefully several times, the whole can be easily sectioned The structure is essentially as in the adder-tongue, but the epiderular, and the fibro-vascular bundles are better developed They are like those of the stem, but soround tissue is composed, as in the leaves we have studied, of chlorophyll-bearing, loose cells, rather more compact upon the upper side (In the majority of dicotyledons the upper surface of the leaves is nearly or quite destitute of breathing pores, and the cells of the ground tissue below the upper epider what is called the ”palisade-parenchyma” of the leaf)
[Illustration: FIG 95--_A-D_, successive stages in the development of the flower of _Capsella_, 50 _A_, surface view _B-D_, optical sections _s_, sepals, _p_, petals _an_ sta anther, 180 _sp_ spore rown anther _sp_ pollen spores, 50
_F'_, four young pollen spores, 300 _F”_, pollen spores ger pistil in optical section, 25 H, cross-section of a somewhat older one _ov_ ovules _I-L_, development of the ovule _sp_ embryo sac (rown ovule, 150 _Sy_ _Synergidae_ _o_, egg cell _n_, endosperm nucleus _ant_ antipodal cells _N-Q_, development of the embryo, 150 _sus_ suspensor]
The shepherd's-purse is an admirable plant for the study of the developiosperms To study this, it is only necessary to teaze out, in a drop of water, the tip of a racelass, examine with a power of froes it is best to treat with potash, which will render the young flowers quite transparent The young flower (Fig 95, _A_) is at first a little protuberance composed of perfectly similar small cells filled with dense protoplasm The first of the floral leaves to appear are the sepals which very early arise as four little buds surrounding the young flower axis (Fig 95, _A_, _B_) The sta at first entirely si sepals The petals do not appear until the other parts of the flower have reached some size, and the first tracheary tissue appears in the fibro-vascular bundle of the flower stalk (_D_) The carpels are more or less united from the first, and fores turned in (_D_, _gy_) This cup rapidly elongates, and the cavity enlarges, beco coma develop The ovules arise in two lines on the inner face of each carpel, and the tissue which bears therows out into the cavity of the ovary until the two placentae meet in the95, _H_)
The stamens soon show the differentiation into filament and anther, but the former remains very short until immediately before the flowers are ready to open The anther develops four sporangia (pollen sacs), the process being very similar to that in such pteridophytes as the club_E_, _F_) contains a central mass of spore mother cells, and a wall of three layers of cells The spore mother cells finally separate, and the inner layer of the wall cells becomes absorbed much asin the fern, and the mass of ium Each one now divides in precisely the saymnosperms, into four pollen spores The anther opens as described for _Erythroniu ovaries, ovules in all stages of development may be found, and on account of their small size and transparency, show beautifully their structure Being perfectly transparent, it is only necessary toovule (_I_, _J_) consists of a central, elongated body (nucellus), having a single layer of cells enclosing a large central cell (the macrospore or embryo sac) (_sp_) The base of the nucellus is surrounded by two circular ridges (i, ii) of which the inner is at first higher than the outer one, but later (_K_, _L_), the latter grows up above it and completely conceals it as well as the nucellus One side of the ovule grows much faster than the other, so that it is cou 95, _L_) This opening is called the ”micropyle,” and allows the pollen tube to enter
The full-grown embryo sac shows the sae 276), but as the walls of the full-grown ovule are thicker here, its structure is rather difficult to95, _F_) that hold the pollen spores whichout the pollen tube By carefully opening the ovary and slightly crushi+ng it in a drop of water, the pollen tubethe stalk of the ovule until it reaches and enters thefruits should be selected, and the ovules carefully dissected out and mounted in water, to which a little caustic potash has been added The ovule will be thus rendered transparent, and by pressing gently on the cover glass with a needle so as to flatten the ovule slightly, there is usually no trouble in seeing the e in the upper part of the e more firmly it can often be forced out upon the slide The potash should now be re paper, and pure water run under the cover glass
The fertilized egg cell first secretes a membrane, and then divides into a row of cells (_N_) of which the one nearest the ed The cell at the other end next enlarges and becoht cells This globular ether with the cell next to it, is the e no further part in the process, and being known as the ”suspensor” Later the embryo becomes indented above and fors of the cotyledons The first root and the stem arise from the cells next the suspensor