of cells and tissues. Under special condidons, cell-division brings about the 

 healing of wounds, breaks or injuries (see page 228). And through regenera- 

 tion cell-di\ision may give rise to new indniduals; a fraction of a worm or a 

 starfish, for example, may become a new indi\'idual. 



Regeneration and Reproduction^ Fruit growers propagate new lots of 

 individuals by setting out slips, or cuttings, from especially desirable plants, 

 and having them take root. Even where culti\ated plants bear seeds, it is 

 sometimes more practicable to use this vegetative propagation than to depend 

 upon seeds. The strawberry and other common plants normally split them- 

 selves into multitudes of individuals through vegetative propagation (see 

 illustration, p. 372). A long shoot of forsythia or of wisteria may droop to the 

 ground and take root; later the connection with the parent plant dies away. 

 The horticulturist regularly makes use of this process, as in "layering" rasp- 

 berries: he brings a stem over and fastens it in contact with the ground 

 until it establishes itself by means of roots (see illustration, p. 373). 



For all practical purposes, new individuals do arise from cell-division in 

 budding, regeneration, and other growth processes. But we commonly dis- 

 tinguish between growth, which means an increase in the quantity of Hving 

 matter, and reproduction, which means the bringing of new individuals into 

 being. Moreover, we usually think of reproduction as an event or process 

 that separates one generation from the next. 



How Is Reproduction Different from Growth? 



Spores and Cysts" If the conditions for growth become unfavorable, 

 some species of protozoa form a thick cell-wall inside of which the protoplasm 

 may remain indefinitely inactive. In this incased state, or cyst, the animal 

 may resist drought or frost, or even the digestive juices of some stomach into 

 which it may get. The cyst is thus a resting stage in which animals can survive 

 adverse conditions. 



Among the simplest plants unfavorable conditions lead to the formation 

 of a somewhat similar resting stage. Yeast cells, for example, divide the 

 protoplasm into four parts, each of which puts out a thickened wall (see illus- 

 tration opposite). Such a special cell is called a spore, and is able to resume 

 growth when conditions are again favorable. Spores are produced in nearly 

 all species of plants and in some animals. They are also usually formed in 

 large numbers and are very resistant to unfavorable conditions. In the spore 

 stage some kinds of bacteria cannot be killed by boiling water. 



One class of protozoa, the Sporozoa (see Appendix A), consists of parasitic 

 forms which reproduce by means of spores. These special cells result from 



iSee No. 4, p. 395. -See No. 5, p. 395. 



370 



