76 



A TEXTBOOK OF THEORETICAL BOTANY 



and attached to its margin. This sheath becomes impregnated with wall 

 material and is soon indistinguishable from the wall itself. Its upper end 



Oosphere 



■%f^A<k-^?'. 



Fig. 58. — Coleochaete scutata. Female thallus with 

 oogonia. In this common species, which is 

 dioecious, no filaments are formed. Oogonia and 

 antheridia are developed at the apices of radial 

 cell rows, but are outgrown by the vegetative cells 

 and remain embedded in the thallus, often forming 

 concentric zones. 



becomes perforated, and a stream of cytoplasm passes out through the 

 sheath, becoming homogeneous and solid, and forming the hair. Usually 

 these hairs project at right angles to the surface of the thallus : later they 

 may break off leaving only the sheath behind. 



Asexual Reproduction 



Asexual reproduction occurs chiefly in spring and early summer. Any 

 cell of the thallus is capable of functioning as a zoosporangium, but most 

 frequently it is the terminal cell of a filament which is involved. The contents 

 round up to produce a single large, ovoid, biflagellate zoospore which 

 frequently has a single laterally placed chloroplast ; it has no eye spot. After 

 swimming for a while it settles down and divides. If the first division is 

 horizontal the upper cell forms a hair, while the lower cell forms an embryo 

 thallus ; if the division is vertical each segment grows out laterally as a 

 thallus cell, but in either case the hair formation takes place at a very early 

 stage. 



Aplanospores, i.e., non-motile spores, with fairly thick walls, may be 

 developed from any cell of the thallus, one being produced from each cell. 



