130 CMorophycese 



of certain of the Conjugatse the Green Algae do not apparently possess a protein 

 pigment soluble in water such as the phycocyanin of the Myxophycese or the 

 closely related red pigment of the Rhodophycese. The chloroplasts may be 

 solitary or numerous, with entire margins, or with margins so lobed and incised 

 as to present an almost infinite variety of form. In most families they are 

 parietal, but the elaborate chloroplasts of the majority of Desmids, and those of 

 Zygnema, Prasiola, etc., are axile. In some genera the parietal chloroplasts 

 are ribbon-like or band-like, being wound spirally round the interior of the 

 cell- wall (Spirogyra and some species of Spirot&nia), and in others the axile 

 chloroplast is furnished with spirally twisted ridges after the manner 

 of screw threads (some species of Spirottvnia). It is difficult to account 

 for the extraordinary diversity in the form of chromatophores, especially as 

 that diversity may be found even in the members of one family, as in the 

 case of the Desmidiaceae. They all perform the same function, and yet Algse, 

 with chromatophores entirely different both in form and disposition, live in 

 association under precisely the same conditions. Oltmanns' comparison 

 of the variability in the shape of chromatophores with the variability in the 

 shape of the leaves of higher plants is a very apt one. In the lower types, 

 and in a few of the higher types, the chloroplasts are sometimes very massive, 

 in consequence of which it is difficult to ascertain their exact shape and 

 limitations by direct observation; but very often they occupy only a re- 

 latively small part of the cytoplasm, in which case they are mostly clear 

 and well-defined. In a few cases the chlorophyll is diffuse, no part of the 

 cytoplasm being specially demarcated for the lodgment of the colouring 

 matter. This has been definitely shown by Timberlake ('01) to be the 

 case in Hydrodictyon reticulatum, and it seems probable that a similar 

 condition may exist in other Green Alg in which the chloroplasts are obscure. 



In the terminal cells of piliferous branches, and also in the cells of 

 rhizoids, chloroplasts may be reduced or entirely wanting. To some extent 

 the chloroplasts are characteristic of the different families and genera. 



In the chloroplasts of most Green Algae are pyrenoids, which consist 

 of a central crystalloidal portion of protein surrounded by a starchy envelope 

 of variable magnitude. The most important part of the pyrenoid is the 

 central protein portion, which is of the nature of an aleurone grain, and 

 in many of the lower types the envelope of starch either does not exist 

 or is reduced to a minimum. In the higher types and especially in the 

 Conjugata?, the accumulation of starch in the form of rounded or angular 

 grains around the protein is often so great that the protein material is with 

 difficulty detected in the living cell. The protein centre of the pyrenoid 

 stains with practically all nuclear stains, and very deep staining may be 

 obtained by acid-fuchsin after fixation with picro-sulphuric acid or an alcoholic 

 solution of corrosive sublimate. 



