806 ECOLOGY 



nules differ from most other sorts in the relative absence of protective 

 structures. 



Vegetative reproduction in the algae. The simplest form of repro- 

 duction is by fission or cell division, and is well illustrated by various 

 unicellular algae, in which the mature cell divides, producing two or 

 more new cells, which, whether cohering or becoming detached, repre- 

 sent new plant individuals (figs. 858, 4, 34). In most unicellular species 

 the entire body takes part in propagation, so that the disappearance of 

 the adult organism necessarily is coincident with the development of its 

 progeny, though the original cell wall may remain for some time. Simple 

 fission of this character is the only form of reproduction in unicellular 

 blue-green algae and in some green algae (as Pleurococcus) . 



Many algae are filamentous, since division takes place only in parallel 

 planes, and since the newly formed cells cohere (figs. 859, 6). While 

 such filaments have a certain individuality, the cells, at least in the 

 lower algae, are essentially independent, and therefore to be regarded 

 as potential individuals; hence as in unicellular forms, cell division may 

 here be called vegetative reproduction, whether or not the new cells 

 become detached from the old. True filamentous forms differ from 

 unicellular species in that the adult does not necessarily disappear as its 

 vegetative progeny develops. In the higher filamentous algae, cell 

 coherence becomes a fixed feature, and the individuality of single cells 

 is less marked, so that commonly a filament as a whole rather than one 

 of its cells is regarded as a plant. Here the propagules rarely are single 

 cells, but rather pieces of filaments that become detached. Even with- 

 out such detachment, however, filamentous algae (such as Spirogyra) 

 by continued elongation may spread so as to fill a pond in a relatively 

 short time; branched forms like Cladophora (fig. 63) would seem partic- 

 ularly suited for rapid propagation of this sort. In many blue-green 

 algae the filaments break in rather definite places, the limits of the new 

 filaments, the so-called hormogonia, being determined by cells differing 

 from the rest, and known as heterocysts (fig. 8). 



Vegetative reproduction in bacteria, fungi, and lichens. Bacteria 

 reproduce only by fission, yet they increase more rapidly than does any 

 other group of plants. The possibility of rapid vegetative increase 

 among the fungi is well illustrated by the growth of molds in moist 

 chambers, where the hyphae spread quickly in all directions from the 

 original center (fig. 1078). Often the fungus mycelium dies at its 

 original growth center, perhaps because of the exhaustion of its food 



