356 



BOTANY 



PART II 



Class IX 

 Chlorophyeeae ( 



8, 25, 26 



) 



When the green Conjugatae and Heterocontae are separated there 

 remains the hxrge natiu-al group of the Chlorophyeeae, inckiding 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 (Oedogonmin, Vaucheria) the 

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

 from the typical simple form. 



Of the five orders included in the Chlorophyeeae 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 re- 

 production is usually effected by 

 the conjugation of gametes which 

 / 9 \ / resemble the zoospores. In all 



the groups, except the Protococ- 

 cales, isogamy is replaced by 

 oogamy in the higher forms. 



chr 



Order 1. Volvocales. — Typical re- 

 presentatives of tiiis order are charac- 

 terised by the cilia being retained by 

 their cells in the vegetative stage ; the 

 P}j Vlia**'^ plants are therefore motile. Each cell 



Fig. 273. - 1. Chhanydomonm anoulosa. (AftiT 'las a nncleus and a chloroplast. The 

 j)iLL.) g, cilia ; v, vacuole ; k, uucUms ; ckr, Volvocales thus resemble the Flagel- 

 chroinatophores ; pv, pyronoid. Si. I'olyloma y^^^.^^ go„ie forms of which, such as Poh/- 

 uvella (after Danoeard) ; a, eye-spot. (From ^^ ;,„,.,-,_ ^^j j^^-, „„ ^j^^ ^^^i of tljeh- 



Oltmanns Algae.) n i. ' i i i a ■ ^\ r 



cell structure be placed in the lornier 



group, but differ }>y the absence of a cell wall and of sexual reproduction. 



ChlcDtiydovwiian (Fig. 273) and Jlacmalococcus (Fig. 274) are widely distributed 



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



closely applied to the i)rotoplast, at the anterior end of which two cilia and a red 



