242 PART III. — VEGETABLE PHYSIOLOGY. 



This fact is also illustrated in the Cinchonas. When the3e plants 

 are cultivated in European greenhouses they produce no quinine, 

 nor do they when grown at either too high or too low elevations 

 in their native Andes ; on the other hand it has been found that 

 by a proper system of cultivation in a highly favorable climate, 

 such as the Neilgherry Hills of India, they may be made to yield 

 three or four times the quantity of alkaloids that they do in the 

 wild state, even in the portions of the Andes most favorable to 

 the growth of these trees. 



It is well known also that the potato tuber, when left to grow 

 exposed to the full sunlight, develops a poisonous principle, sola- 

 nine, which, when grown normally under ground, it does not 

 possess. The steps, therefore, in the process of destructive 

 change, vary within certain limits, in accordance with varying 

 conditions or with various habits and inherited characteristics 

 of the plant. 



The wastes of the plant which are carried to the outside, or 

 excreted, are chiefly gaseous substances, or vaporizable liquids. 

 For example, carbon-dioxide is returned to the air, water escapes 

 as watery vapor, and portions of the volatile oils are gotten rid 

 of in the same way. Most other wastes are retained in the 

 interior of the plant, sometimes as deposits in cell-walls, some- 

 times stored in secretion cells, laticiferous tissues, secretion 

 reservoirs, etc. Silica, however, and wax are deposited in the 

 cuticle or at the surface of plants, and some calcium carbonate 

 is carried out in solution in the water discharged from the water- 

 pores of some plants. 



Influence of Temperature on the Life of the Plant. 

 The vital processes of the plant can only go on within certain 

 limits of temperature. If it be too low, the seed will not germi- 

 nate or the bud unfold, and if it be increased beyond certain 

 limits, life is at first suspended and then destroyed. For each 

 species of plant there is a minimum temperature below which 

 activity ceases, an optimum temperature at which its activities 

 are greatest, and a maximum temperature which cannot be 

 exceeded without stopping the vital processes. These tempera- 

 tures differ for different plants. Some, as the Red-snow plant of 

 the Arctic regions, thrive at a temperature very near the freezing- 

 point while others, as the Vanilla plant, cannot flourish except 

 in the tropics. 



