398 BOTANY PART n 



CLASS IX 

 Chlorophyceae ( l> " 28 ' 38 ) 



When the green Conjugatae and Heterocontae are separated there 

 remains the large natural group of the Chlorophyceae, including several 

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

 or in damp situations ; some large forms occur on the sea coast but 

 do not contribute to the plankton. 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,, Vaucherici) the swarm-spores are of more 

 complicated structure, but can be derived from the typical simple 

 form. 



The swarm-spores exhibit phototactic movements by means of which they 

 reach favourable conditions of illumination for their germination. 



In all the orders sexual reproduction is usually effected by the 

 conjugation of gametes which resemble the zoospores. In all the 

 groups, except the Protococcales, isogamy is replaced by oogamy 

 in the higher forms. The reduction division so far as is known takes 

 place on the germination of the zygote. 



Of the five orders included in the Chlorophyceae the Volvocales 

 stand 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. 



Order 1. Volvocales 



Typical representatives of this order are characterised by the cilia being retained 

 by thejr cells in the vegetative stage ; the plants are therefore motile. Each cell 

 has a nucleus and a chloroplast. The Volvocales thus resemble the Flagellata. 



Chlarnydomonas (Fig. 333) and Haematococcus (Fig. 332) are widely distributed 

 forms consisting of free-swimming cells ( 29 ). In the former the cell membrane is 

 closely applied to the protoplast, at the anterior end of which two cilia and a red 

 eye-spot are situated ; in the latter genus the membrane is separated from the 

 protoplast by a gelatinous layer except at the anterior end. Haematococcus 

 pluvialis occurs commonly in puddles of rain-water, and, like Chlamydomonas 

 nivalis, which gives rise to "red-snow" and occurs on snow in the Alps, etc., 

 is characterised by the presence of a red pigment (haematochrome) in the cells. 



