Part 4 (1/2)

There are various genera of the pond scu in the form of the chloroplasts and also in the position of the spores Of these19, _A_), with two star-shaped chloroplasts in each cell, and _Mesocarpus_ (Fig 19, _B_, _D_), in which the single chloroplast has the form of a thin median plate (B shows the appearance fro the thickness of the plate) _Mesocarpus_ and the allied genera have the spore for cells leaving them

[Illustration: FIG 20--Forms of Desmids _A_, _B_, _Closterium_

_C_, _D_, _D'_, _Cosmarium_ _D_, and _D'_ show the process of division _E_, _F_, _Staurastrum_; _E_ seen from the side, _F_ from the end]

Evidently related to the pond scu for the20) They are confined to fresh water, and seldom occur in masses of sufficient size to be seen with the naked eye, usually being found associated with pond scums or other filamentous forms Many of thethewater plants, especially the bladder weed (_Utricularia_) and other fine-leaved aquatics

The desmids include the most beautiful exa extreant outlines

The cell shows a division into two parts, and is often constricted in the e chloroplast of peculiar form The central part of the cell in which the nucleus lies is colorless

Ayra_, are various species of _Closteriunizable at once by their crescent shape The cell appears bright green, except at the ends and in the e chloroplast in each half is coitudinal plates, united at the axis of the cell

Several large pyrenoids are always found, often forh the central axis At each end of the cell is a vacuole containing s movement

The des slowly over the slide as we examine them, but the mechanism of these movements is still doubtful

In their reproduction they closely resemble the pond scums

ORDER IV--_Siphoneae_

The _Siphoneae_ are algae occurring both in fresh and salt water, and are distinguished fro the form of a tube, undivided by partition walls, except when reproduction occurs The only common representatives of the order in fresh water are those belonging to the genus _Vaucheria_, but these are to be had almost everywhere They usually occur in shallow ditches and ponds, growing on the botto where the water is deeper They forreen, felted reen felts” Sos An examination of one of the masses shows it to be made up of closely matted, hair-like threads, each of which is an individual plant

In transferring the plants to the slide for microscopic examination, they must be handled very carefully, as they are very easily injured Each thread is a long tube, branching soyra_ or _Cladophora_ If we follow it to the tip, the contents here will be found to be denser, this being the growing point By careful focusing it is easy to show that the protoplas the wall, the central cavity of the tube being filled with cell sap In the protoplaser or slobular bodies (_ol_) These latter are drops of oil, and, when the filaether, and for plant, but by treat, numerous very small nuclei may be demonstrated

[Illustration: FIG 21--_A_, _C_, successive stages in the developreen felt (_Vaucheria_) _an_ antheridiuoniu cell _F_, a ripe spore _G_, a species in which the sexual organs are borne separately on the main filament

_A_, _F_, 150 _G_, 50 _cl_ chloroplasts _ol_ oil]

When the filaround, or at the bottom of shalloater, the lower end is colorless, and for it to the earth These rootlets, like the rest of the filament, are undivided by walls

One of the commonest and at the same ti 21, _A_, _F_) The plant multiplies non-sexually by branches pinched off by a constriction at the point where they join theconstricted and separating into several parts, each one constituting a new individual

The sexual organs are forenizable

The first sign of their develop out at right angles to the main filament This branch becomes club-shaped, and the end somewhat pointed and more slender, and curves over This slender, curved portion is almost colorless, and is soon shut off from the rest of the branch It is called an ”antheridium,” and within are produced, by internal division, nurows, its contents beco in number and size About the time that the antheridium beco 21, _B_, _og_) These are the young oogonia, which rapidly increase in size, assu an oval form, and become separated by walls froonia contain a great deal of chlorophyll, appearing deep green

When ripe, the antheridiues the spermatozoids, which are, however, so very sest lenses They are little oval bodies with two cilia, whichwith iodine

[Illustration: FIG 22--_A_, non-sexual reproduction in _Vaucheria sessilis_ _B_, non-sexual spore of _V geonia, which at first are uniformly colored, just before maturity show a colorless space at the top, from which the chloroplasts and oil drops have disappeared (_D_), and at the same time this portion pushes out in the form of a short beak Soon after the wall is absorbed at this point, and a portion of the contents is forced out, leaving an opening, and at the sa contents contract to for 21, _E_, _o_) Alonium opens, the spermatozoids collect about it and enter; but, on account of their g cell, or to deter cell becomes almost at once surrounded by a wall, which rapidly thickens, and forreen color, beco colorless, with a few reddish brown specks scattered through it (_F_)