Part 14 (1/2)

The growing tissue (ca the phloem from the wood, is made up of cells quite like those of the phloee, except that their walls arecells These cells (_B_, _caed in radial rows and divide, les to the radii of the stee the cells undergo is more marked They become of nearly equal dia at the sa the xyles are in contact, the reason of the sharply marked line seen when the stem is examined with the naked eye is obvious On the inner side of this line (_I_), the wood cells are comparatively small and much flattened, while the walls are quite as heavy as those of theon the outer side of the line The srowth ceased at the end of the season, the cells becorowth coain, the first wood cells fororous at this tier cells is softer and lighter colored than that forrowth

The wood isno vessels formed except the first year These tracheids are characterized by the presence of peculiar pits upon their walls, best seen when thin longitudinal sections are76, _D_, _p_) appear in this view as double circles, but if cut across, as often happens in a cross-section of the steles to the radius (tangential), they are seen to be in shape soh the bottom They are formed in pairs, one on each side of the wall of adjacent tracheids, and are separated by a very delicate membrane (_F_, _p_, _G_, _y_) These ”bordered” pits are very characteristic of the wood of all conifers

The structure of the root is best studied in the seedling plant, or in a rootlet of an older one The general plan of the root is much like that of the pteridophytes The fibro-vascular bundle (Fig 75, _M_, _fb_) is of the so-called radial type, there being three xyle with asThis regularity becorows older by the for like that in the steia is on the whole much like that of the club mosses, and will not be examined here in detail The roups of four in precisely the same way as the spores of the bryophytes and pteridophytes, and by collecting the , and crushi+ng the sporangia in water, the process of division may be seen For more careful examination they may be crushed in a entian violet This mixture fixes and stains the nuclei of the spores, and very instructive preparations may thus be made[11]

[11] See the last chapter for details

[Illustration: FIG 77--Scotch pine (except _E_ and _F_) _A_, end of a branch bearing a cluster offemale flowers (?), natural size _C_, a scale froia (_sp_); 5 _D_, a single ripe pollen spore (etative cell (_x_), 150 _E_, a similar scale, from a female flower of the Austrian pine, seen froiu the scale (_sc_) attached to the back _G_, longitudinal section through a full-grown ovule of the Scotch pine _p_, a pollen spore sending down its tube to the archegonia (_ar_) _sp_ the prothalliu up the eoniu 77, _D_) are oval cells provided with a double wall, the outer one giving rise to two peculiar bladder-like appendages (_z_) Like the microspores of the smaller club mosses, a small cell is cut off from the body of the spore (_x_) These pollen spores are carried by the wind to the ovules, where they geriule layer of cells in es which have to do with opening the pollen sac

We have already exaiurown ovule the enerally known as the ”embryo sac,” is completely filled with the prothallium or ”endosperonia are for cell is very large, and appears of a yellowish color, and filled with large drops that give it a peculiar aspect There is a large nucleus, but it is not always readily distinguished frooniu, but does not project above the surface of the prothalliureat numbers, and many of them fall upon the female flowers, which when ready for pollination have the scales somewhat separated The pollen spores now sift down to the base of the scales, and finally reach the opening of the ovule, where they germinate No sper in this respect from all pteridophytes The pollen spore bursts its outer coat, and sends out a tube which penetrates for so veryat the expense of the tissue through which it grows After a tirowth ceases, and is not resuonia is nearly complete, which does not occur until more than a year from the time the pollen spore first reaches the ovule Finally the pollen tube penetrates down to and through the open neck of the archegoniues can only be seen by careful sections through a number of ripe ovules, but the track of the pollen tube is usually easy to follow, as the cells along it are often brown and apparently dead (Fig 77, _G_)

CLassIFICATION OF THE GYMNOSPERMS

There are three classes of the gymnosperms: I, cycads (_Cycadeae_); II, conifers (_Coniferae_); III, joint firs (_Gnetaceae_) All of the gy to the second order, but representatives of the others are found in the southern and southwestern states

The cycads are palle trunk crowned by a circle of corown for ornament in conservatories, and a few species occur native in Florida, but otherwise do not occur within our liy leaf of a cycad (_Cycas_), with ko_,_C_, branch of hea_), with a ripe cone, 1 _D_, red cedar (_Juniperus_), 1 _E_, _Arbor-vitae_ (_Thuja_), 1]

The spore-bearing leaves usually for somewhat in structure those of the horse-tails, but one of the commonest cultivated species (_Cycas revoluta_) bears the ovules, which are very large, upon leaves that are in shape78, _A_)

Of the conifers, there are nureen trees There are two sub-orders,--the true conifers and the yews In the latter there is no true cone, but the ovules are borne singly at the end of a branch, and the seed in the yew (_Taxus_) is surrounded by a bright red, fleshy integu shrub, occurs sparingly in the northern states, and is the only representative of the group at the north The European yew and the curious japanese _Gingko_ (Fig 78, _B_) are sometimes met with in cultivation

Of the true conifers, there are a number of families, based on peculiarities in the leaves and cones Some have needle-shaped leaves and dry cones like the firs, spruces, he 78, _C_) Others have flattened, scale-like leaves, and78, _D_) and _Arbor-vitae_ (_E_)

A few of the conifers, such as the tamarack or larch (_Larix_) and cypress (_Taxodium_), lose their leaves in the autureen”

The conifers include soest of trees Their timber, especially that of some of the pines, is particularly valuable, and the resin of so the giant red-woods (_Sequoia_) of California, the largest of all American trees

The joint firs are co the dimensions of trees They are found in various parts of the world, but are few in number, and not at all likely to be met with by the ordinary student Their flowers are rather ymnosperms, and are said to show soiosperms

CHAPTER XV

SPERMAPHYTES

CLass II--ANGIOSPERMS