PHYSIOLOGY OF GROWTH AND CONFIGURATION 



rosette close to the ground. This species is thus so strongly influenced by the 

 climatological conditions of high altitudes that it assumes the typical form of 

 an alpine plant even in the first generation under these conditions. 



These examples show how readily the forms of many plants become altered 

 by changed external conditions. Even in the first generation this influence of 

 the surroundings may be very marked, and when the new set of conditions is 

 effective throughout a number of generations the resulting changes may be in- 

 herited. Such inherited characteristics may then be retained throughout a num- 

 ber of generations, notwithstanding further environmental changes.' Fig. 164 



represents spruce seedlings three years old, 

 all grown under identical conditions, but 

 from seed that came from different regions. 

 Seed from trees growing under favorable 

 conditions, at relatively low altitude (800 

 m.), produced very large plants (Fig. 164, 

 2), but seed from trees of the same geographic 

 region but growing at higher altitudes (1600 

 m.) produced much smaller plants (Fig. 

 164, i). The plants obtained from seed 

 that grew in Finland (Fig. 164, 3) were much 

 smaller than any of the others. 



Practical as well as theoretical impor- 

 tance is attached to the principle of heredity 

 just illustrated, for to obtain a good agricul- 

 tural crop not only must the soil be well 

 cultivated and fertilized but seed of a good 

 stock or strain must be used also. 



Scientists have not been satisfied with 

 studying the influence of external conditions 

 in the control of form and structure of 

 plants, but they have also been interested in 

 discovering , the genetic relationships that 

 exist between plant organs. Until very re- 

 FiG. 165.— Formation of potato cently problems of this sort have been 



tubers above the soil, on darkened por- \ 1 • 1 1 1 1 r • i 



tion of the stem. (After Vochting.) attacked exclusively by the method of simple 



observation. From such morphological ob- 

 servations, the plant body (in the case of vascular plants) is considered as made up 

 of three primary parts or organs, roots, stems and leaves; all other organs not at 

 first recognizable as roots, stems or leaves are regarded as modifications of one of 

 these three types. Thus floral parts are considered as modified leaves. Potato 

 tubers are a special kind of short, thick, underground stem, since they are formed 

 on subterranean stem-branches and not on the roots. The so-called potato- 

 eyes are dormant buds with embryonic leaves, which furnish additional 

 evidence that the tuber is really a kind of stem. This conception arose as 



' Demoore, J., La m^moire organique. Bull. Soo. Roy. Sci. M^d. et Nat. Bruxelles 65 : 28-40. 1907. 



