118 PKACTICAL BOTANY 



(Fig. 265). The result of fertilization is to cause the egg of 

 the ovule to develop into an embryo. One of the first steps 

 of embryo formation is shown in Fig. 267. The fertilization of 

 the egg within the ovule may result from the proper placing 

 of a single pollen grain, but the result is more certain if there 

 are several grains. 



109. Advantages of reproduction by seed. As has already 

 been shown (Sects. 33 and 75-80), reproduction may readily 

 be accomplished by buds produced on roots, stems, or leaves, 

 vegetative reproduction. This method is much quicker than 

 that by the agency of seed, as is well shown in the case of 

 the potato. It would seem that sexual reproduction, repro- 

 duction by means of seed, a more complicated process, would 

 hardly have originated unless on the whole it were of advan- 

 tage to plants. 1 It is evidently desirable for the continuation 

 of the various kinds of plants to have such a comparatively 

 portable, heat-, cold-, and drought-resisting structure as the 

 seed to disseminate plants over large areas and to maintain 

 plant life under unfavorable conditions. But some botanists 

 have been led to think also that sexual reproduction is of 

 distinct advantage to plants by giving them greater vigor 

 than is secured by long-continued vegetative reproduction, as 

 in the case of potatoes grown for years by planting the tubers. 

 It is also of advantage to the plant to reproduce by means of 

 seed, because this secures variations in the offspring which 

 may result in greater fitness to meet the conditions of its 

 existence (Chapter XXIII). 



110. Ecology. Plant ecology (from two Greek words meaning 

 "house" and "discourse") is the subdivision of botany which 

 discusses the relations of the plant to its surroundings. Defin- 

 ing the subject more in detail, it may be said that ecological 

 botany treats of the effects upon plants of the various forces 

 and forms of energy, such as gravity, heat, light, electricity, 

 currents of air and water, as well as of the effects of chemi- 

 cal elements and compounds. It also comprises the study of 



1 For a brief account of the beginnings of sex in plants see Chapter XIII. 



