8io 



ECOLOGY 



in their unicellular nature, In their production by specialized organs, and 

 in their capacity for endurance, which often is increased by the presence 

 of thick protective walls; however, hard and fast lines are not to be 

 drawn, since some spores are incapable of enduring severe periods, while 

 gemmae may be unicellular or borne by special organs. 



Reproductive structures generally have one or more of three char- 

 acteristics : capacity for increasing the number of individuals in a species 

 (which is, of course, the primary feature of reproduction) ; capacity for 

 endurance through severe periods; and capacity for dispersal. Asexual 

 spores are efficient in all three respects, thus contrasting with propagules, 

 which have been seen to be relatively ineffective as disseminules and 

 often unfitted for endurance, though they are the most efficient of aU 

 means of multiplying individuals. Asexual spores occur in nearly all 

 plant groups, though they are unknown in various algae (as in the Con- 

 jugales, Fucales, and Charales), and are practically absent in some 

 higher plants (as in various mosses and in the duckweeds). 



Asexual spores in the algae. — The most representative asexual spores 

 among the algae are the zoospores or swarm-spores, which differ from 

 most spores in being without protective walls, and 

 whose chief distinguishing character is the power 

 of locomotion in water ; usually they move by 

 means of variously arranged cilia, which may be 

 single (as in Botrydium, fig. 92), two (as in Hy- 

 drodictyon, fig. 1121), four (as in Ulothrix, fig. 

 1 133), or many (as in Oedogonium and Vaucheria, 

 figs. 76, 96). While most characteristic of green 

 algae, swarm-spores occur in some of the brown 

 algae (as in Edocarpus, fig. 121). Zoospores are 

 among the most efficient of reproductive struc- 

 tures, partly because commonly they are produced 

 in large numbers, but particularly because they 

 differ from almost all other disseminules in ex- 

 hibiting directive dispersal. For example, they are 

 prophototactic, hence they usually move to a well- 

 hghted situation where they may germinate into 

 new plants under favorable conditions. The lack of protective walls is 

 hardly a disadvantage, since zoospores are not exposed to transpiration, 

 nor are they obliged to live over unfavorable seasons. In the red algae 

 true zoospores are wanting, though a few species have spores that exhibit 



Fig. 1121. — A zoo- 

 spore or swarm spore 

 of the water-net (Hy~ 

 drodictyon); note the 

 two cilia which effect 

 locomotion; highly 

 magnified. — After 



TiMBERLAKE. 



