ON THE STUDY OF ADAPTATION IN PLANTS. 187 



As regards the second kind of influence — that of growth 

 — it will be well to remember in the first place the division 

 of the different phases of growth, which Sachs 1 has made. 

 He distinguishes two periods in the development of the 

 organs : — 



I. Morphological period — 



(1) Origin of the organs according to number and 



position. 



(2) Embryonic growth of the organs, morpho- 



logical configuration, position in the bud. 



II. Physiological-biological period — 



(3) Extension of the organs up to reaching their 



definitive size. 



(4) Internal development of the tissue, and the 



maturation of the plant organs. 



The influence of external factors concerns only the 

 physiological-biological periods, but its influence may, never- 

 theless, be a very striking one. And here it will be well to 

 quote a few examples. 



The characteristic habits of many Alpine plants are 

 generally known ; the low growth of the shoot-axis, and 

 the relatively energetic development of the subterranean 

 parts. Gaston Bonnier 2 investigated the effect of an ele- 

 vated Alpine position (more than 2000 metres) on a number 

 of plants inhabiting the lowlands. Out of 203 of the plants 

 examined, 123 remained living in the high regions, and 

 they exhibited the following changes : the whole plant was 

 smaller, and its stems grew nearer the ground ; some had 

 undergone such striking changes that they could only with 

 difficulty be recognised, such as the Heliauthus tuberosum, 

 which had developed on the ground a rosette densely 

 clothed with hairs, instead of the ordinary elongated shoot. 

 The leaves remained smaller and thicker in proportion to 

 the surface, and they were of a deeper green than those 

 from lowland-grown specimens, and some even acquired 

 a brownish-red colouration. The subterranean parts 



1 Flora, 1895, p. 577. 

 Revue de botanique, ii. 513. 



