THE GREEN ALGJE 



481 



a woolly mat which suggests the name wool-weeds as appro- 

 priate for the group. A differentiation of the plant-body 

 appears in a specialization of the lowest cell as an organ of 

 attachment. This, therefore, is analogous to a root while 

 the remaining green part has the function of a shoot. But 



v-^'r 



Fig. 313. — Wool-weed {Ulothrix zonata. Wool-weed Family, Ulotrichacew). 

 A, young plant with basal cell (;•) serving as a root-like organ of at- 

 tachment, 3?&. B, part of plant with escaping swarm-spores. C, single 

 swarm-spore. D, formation and escape of gametes. E, free-swimming 

 gametes. F, gametes conjvigating. G, conjugation complete. H, zy- 

 gote. /, zygospore after a period of rest. K, zygospore after division of 

 its protoplast into swarm-spores. B-K, ^m, (Dodel-Port.) — 

 Abundant on submerged rocks, especially in fresh water. 



the resemblance not being one of homology, it will be most 

 accurate for us to name organs of this kind pseudo-roots and 

 pseudo-shoots respectively. Thallus ^ is a general term for 

 any plant-body in which true roots, true stems, and true 

 leaves do not appear. The pseudo-shoot of a wool-weed 

 elongates by fission of its cells, each of which contains a 



1 Thal'lus < Gr. thallos, a young shoot. 



