38 



PLANT MORPHOLOGY 



Summary. The Chlorococcales range from simple isolated cells to 

 complex colonies. In this and succeeding orders nonmotility is the 

 permanent condition of the vegetative cells. Although usually uninu- 

 cleate, frequently these are multinucleate and often contain more than 

 one chloroplast. Colonies are formed by the coming together of free 

 cells (usually zoospores) derived from a single parent cell and there 

 is no subsequent division of vegetative cells. Cell division occurs 

 only in connection with the formation of reproductive cells. Reproduc- 

 tion is accomplished by zoospores, aplanospores, or akinetes, and usually 

 also by isogametes. 



3. Ulotrichales 



The Ulotrichales have been called the representative group of the 

 Chlorophyceae. Most of them live in fresh water but some are marine. 



.Q.-- 



-<3' 



•.--G>.0;: 



K® 



®-. 



'01 



feiO- 



<^ 



iSti 



ABC 



Fig. 27. Ulothrix zonata, vegetative and reproductive stages, X700. A, basal portion of 

 filament, showing holdfast cell and three vegetative cells, each with a single nucleus and a 

 peripheral band-like chloroplast with many pyrenoids; B, formation and escape of zoospores; 

 C, formation and escape of gametes, some of which are pairing. 



A few live in damp places on land. Trichophilus grows inside the hair of 

 the South American sloth. To this order belong 85 genera and approxi- 

 mately 500 species, the principal genera being Ulothrix, Chaetophora, 

 Draparnaldia, Stigeoclonium, Protococcus, Ulva, and Coleochaete. 



Ulothrix. This alga is of widespread occurrence in streams, lakes, 

 and ponds, where it grows attached to objects in the water. A few 

 of its species are marine. The plant body is multicellular, consisting 

 of a simple unbranched filament (Fig. 27 A). The basal cell is elongated 



