THE LIFE CYCLE 333 



The special means by which individuals multiply them- 

 selves asexually are far too numerous and diverse for us to 

 attempt to consider them all here. Stools and stolons and 

 runners and tubers and offsets and bulbs and a dozen 

 kinds of detachable buds, are known to every student of 

 plants. Indeed, many of those plants that have been able 

 to advance into and conquer difficult environments and 

 become dominant in them, (such as the pond-weeds on the 

 bottom in shoal waters, and the grasses and sedges in the 

 fire-swept prairies and marshes) increase mainly asexually, 

 by extensions of the plant body. They still produce seeds, 

 but they hold their ground by continuous and exclusive 

 occupancy of it. Budding and fragmentation and other 

 such methods are common also among the lower animals. 

 This we have observed in the hydra. But all such increase 

 has to do with growth as well as with reproduction. Let us 

 here consider some more specialized reproductive parts and 

 methods, that are more exclusively adapted to reproductive 

 ends. 



Asexual reproductive cells. — When these are formed for 

 dispersal, they are usually called spores. With ordinary 

 spores, as they are com.monly produced upon the aerial parts 

 of plants, we have already become acquainted. These are 

 minute resting cells, usually invested with a protective 

 covering that resists evaporation, and that permits of their 

 being distributed by currents of air. 



Among aquatic thallophytes, both algye and fungi, there 

 occurs at intervals a breaking up of the cell contents into 

 minute naked unicellular reproductive bodies. These are 

 called zoospores or swarm spores. Each zoospore acquires 

 two or four flagella (or sometimes a circlet of cilia), and, 

 escaping out of the old cell wall, it swims about in the 

 water. Finding a suitable situation it attaches itself and 

 begins to develop a new plant body like that of its parent. 



