470 BIOLOGY AND HUMAN LIFE 



344. Spores. The term reproduction is sometimes reserved for proc- 

 esses in which the plant (or animal) gives rise to a specialized cell that 

 can grow into a new individual. The simplest of such specialized cells are 

 the spores. In ordinary cell division new individuals arise, but each 

 of the new cells is just like the mother cell, and this mother cell at the 

 time of division is apparently the same as any growing cell. In the yeast 

 plant, when the conditions for growth are no longer favorable (for ex- 

 ample, extreme temperature and lack of food or of water), the proto- 

 plasm in a cell shrinks up and divides into two parts, and each of these 

 divides again. The original cell wall may become somewhat thickened. 

 In this condition the protoplasm may resist drought or cold indefinitely ; 

 and when conditions are again suitable, it may absorb water, and each of 

 the four units may start a new vegetative series. These special resting 

 cells are spores. Nearly all plants produce spores, and usually in large 

 quantities. Fungi are widely distributed because their numerous and 

 very small spores are easily scattered as dust by the winds, as well as by 

 insects, birds, and other animals. The pollen grains in flowering plants 

 (sect. 35) are spores. In mosses the spores are borne in neat Httle cap- 

 sules on the ends of slender stalks on top of the leafy stem. In ferns 

 we can find groups of tiny spore cases, usually on the under surfaces of 

 the fronds, in the form of dark dots (see Figs. 40 and 41). 



345. Conjugation. In many plants, including fungi and algae, 

 there occurs a process whereby the contents of tv^o cells (that is, 

 the protoplasm) unite to form a nev^ cell that may later start 

 another generation. In the Spirogyra (the common pond-scum, 

 or frog spit) the individual cells are all alike and almost wholly 

 independent of each other. Each cell has chlorophyl and is 

 capable of absorbing what it needs and manufacturing its own 

 food. Lender certain conditions these cells grow in length and 

 divide across the middle so that in the course of a few sunny 

 days in the spring a pond may become covered over with mil- 

 lions of the green threads. Under conditions of darkness and 

 low temperature, growth cannot take place, since chlorophyl 

 cannot then produce food ; but as the threads become tangled 

 in the water two cells lying opposite each other may put forth 

 budlike outgrowths which meet. The protoplasm of one cell 

 then passes over into the other, where they unite and form a new 

 kind of cell (see Fig. 193). This cell is like a spore in being 



