2o8 CHLOROPLASTS OF ALG^ 



lining layer of cytoplasm within the thin cell-wall ; it contains 

 one or more pyrenoids.^ In Cladopliora and (Edogoniimi , on the 

 other hand, the chloroplast is perforated and becomes a network 

 with numerous scattered pyrenoids. In CEdogoniimi (Fig. 117, «) 

 the meshes are elongated and more or less parallel to one another, 

 whilst in Cladophora (Fig. 104, D) the network is irregular and 

 often consists of numerous separate disc-shaped portions, 

 some of which possess pyrenoids. Other peculiarities of the 

 cell-structure of Cladophora are the numerous small nuclei 

 (Fig. 104, D, H.), that are recognisable after careful staining, on 

 the inner side of the chloroplasts, and the thick stratified wall 

 to which this Alga owes its coarse texture ; the former feature 

 is in marked contrast to the single nucleus found in the cells 

 of Ulothrix and especially obvious in Qidogonium (Fig. 117, a, «.). 



The non-septate threads of Vaucheria possess numerous 

 discoid chloroplasts which are lodged in the lining layer of cyto- 

 plasm and lack pyrenoids (Fig. 118, D, c) ; this xA.lga also contrasts 

 with other Chlorophyceje in producing no starch, the excess 

 food being stored as oil. In the Red and Brown Algge (Fig. iig, 

 d, cli.), as well as in Diatoms, the cells contain several chloro- 

 plasts which usually have a more or less lobed outline, and not 

 uncommonly enclose structures resembling pyrenoids ; in these 

 groups the cells generally possess one or two nuclei. 



The most elaborate types of chloroplasts are, however, found 

 in Spirogyra and its allies, which belong to a group of the Green 

 Algffi known as the ConjugatcB, practically confined to fresh-water. 

 Some of these are filamentous like Spirogyra (Fig. 4), whose 

 spiral chloroplasts, one or more to each cell, have already been 

 described (p. 7), and Zvgnema (Fig. 122, F), where the cells 

 contain two star-shaped chloroplasts with a conspicuous pyrenoid 

 [p.) at the centre of each. A large number of the Conjugata; 

 are, however, unicellular forms, named Desmids (Fig. 114), 

 which resemble Spirogyra and Zygnema in their methods of 

 reproduction, but in many cases have even more elaborate 

 chloroplasts. 



The Desmid-cell usually exhibits two symmetrical halves, 



' In the Alga Honnidiiiiii, which occurs very commonly on damp soil, 

 and in which much the same type of chloroplast is encountered (I'ig. no, D), 

 the character of the latter is often very readily recognised. 



