SECT. I 



CRYPTOGAMS 



353 



CLASS IX 

 Chlorophyceae (- -"' 21 ) 



When the green Conjugatae and Heterocontae are separated there 

 remains the natural group of the Chlorophyceae, including several 

 series of genera. The majority of these Algae live in fresh water 

 or in damp situations, but a large number are found in the sea. 

 Their characteristic chloroplasts are of a pure green colour, frequently 

 contain pyrenoids, and nearly always form starch. The asexual 

 swarm-spores are pear-shaped, and in typical forms possess two or 

 four cilia of equal length (on this account the group is sometimes 

 termed Isocontae) and a curved or bowl-shaped chloroplast. In some 

 genera the swarm-spores are replaced by non-motile aplanospores, and 

 in certain of the more advanced genera (Oedogonium, F'aucheria) the 

 swarm-spores are of more complicated structure, but can be derived 

 from the typical simple form. 



Of the five orders included in the Chlorophyceae the Volvocales 

 stands nearest to the Flagellata and, as is also the case with the 

 Protococcales, include unicellular and colonial forms. The Ulotrichales 

 and Siphonocladiales are filamentous ; in the former the filaments are 

 composed of uninucleate, in the latter of large multinucleate cells. 

 The filaments are simple in the lower forms, but branched in the 

 more advanced ones. The thallus of the Siphonales is formed of a 

 single multinucleate cell. 



In all the orders sexual reproduction is usually effected by the 

 conjugation of gametes which re- 

 semble the zoospores. In all the 

 groups, except the Protococcales, 

 isogamy is replaced by oogamy 

 in the higher forms. 



Order 1. Volvocales. Typical re- 

 presentatives of this order are charac- 

 terised by the cilia being retained by 

 their cells in the vegetative stage ; the 

 plants are therefore motile. Each cell 

 has a nucleus and a chloroplast. The 

 Volvocales thus resemble the Flagel- 

 lata, some forms of which, such as Poly- 

 blepharis, might, on the ground of their 

 cell structure, be placed in the former FIG. 282. 1. Chlamydomonas angulosa. (After 



group, but differ by the absence of a DlLL -> ' cilia = v > vacuole *. nucleus ; chr, 



,,,, . i^- chromatophores ; py, pyrenoid. 2. Polytoma 



cell wall and of sexual reproduction. uvdla (after DANGEARD) . a> eye-spot. (From 



Clilamydomonas (Fig. 282) and OLTMANNS' Algae.) 

 Hacmatococcus (Fig. 283) are widely dis- 

 tributed forms consisting of free-swimming cells ( ffl ). In the former the cell 



2A 



