INFLUENCE OF TEMPERATURE. 503 



in vol i. pp- 284 and 307, that it is needless to repeat them here; but it should 

 be noted that the capacity of plants to construct their tissue as need requires, 

 either for aiding transpiration or for protection against excessive evaporation, 

 is very limited. It must also be pointed out that it is very difficult to distinguish 

 clearly between the direct effect of the humidity of the air and the effects of 

 other influences. Heat and light, as vrell as the amount of moisture in the soil, 

 are intimately connected with the humidity of the air, but the relations are 

 difficult to estimate. To a certain extent they are interchangeable, and therefore, 

 in most instances, it is impossible to say which external influence is the cause of 

 any particular alteration in the tissue concerned in transpiration. For the answer 

 to the chief question, whether it is possible for a change in the conditions of 

 life to cause an alteration of form in the sense of an adaptation, it is really a. 

 matter of indifference which influence causes the visible effect. Only here, as in 

 so many other cases, matters are simplified if a certain partiality is permitted in 

 experiments for solving these difficult questions, and if the interwoven influences 

 of soil and climate are treated separately. 



The effect of heat on growing plants was discussed at vol. i. p. 623. It only 

 remains to say here that the formation of starch and other reserve-foods, as 

 weU as the formation of sugar in fruits, is largely connected with heat. Fruits 

 of the same species which ripen under a higher temperature differ greatly in the 

 amount of sugar they contain from those ripening at a lower temperature. It is 

 generally accepted that the size also of the stem, foliage, flowers, and fruit is 

 influenced by heat. The changes which occur when plants in flower, after being 

 for some time in a very warm room are transferred into a cooler room, the 

 other conditions remaining the same, are in particular now recognized. When a 

 large-flowered bulbous plant, e.g. the Belladonna Lily {Amaryllis Belladonna), 

 is transferred to a cold greenhouse after opening its flrst flowers in a warm one, 

 the flowers it here develops at a lower temperature are almost a third smaller 

 than those produced in the warm house. But when the flrst flowers open in 

 the cold, and the later ones in a warm atmosphere, the former remain small 

 and the latter are larger in size. It is important to emphasize this circumstance 

 in order that the phenomenon here exhibited may not be mistaken for another, 

 in case we should be led to think that the flowers of a plant which first unfold 

 are larger than those which succeed them even when there has not been the 

 slightest alteration in the conditions of light, heat, humidity, &c. 



It is particularly instructive, when examining the effect of heat on the form 

 of a species, to compare plants grown in water of different temperatures but 

 under conditions otherwise similar. In mountainous districts the springs on the 

 same mountain slope have a different temperature according to their elevation, 

 and yet the same species of plants may be found growing in springs at the foot 

 and high up on the mountain. Let us take as examples plants of Cardamine 

 amara, Myosotis palustris, Pedicularis palustris, and Veronica Beccabunga. 

 These species grow at the foot of the Patscherkofel, near Innsbruck, in the bed 



