238 BOTANY part i 



for some time in prolonged darkness when the temperature remains 

 constant, thus under these conditions Helianthus tuberosus has been 

 observed to continue its regular daily periods for two weeks, affording 

 an example of the inexplicable occurrence of so-called AFTER-EFFECTS, 

 which are frequently mentioned in a later chapter. 



Still greater is the influence exerted on the life of plants by the 

 alternation of winter and summer, which in the plants of the colder 

 zones has rendered necessary a well-marked winter rest. This is not 

 in reality an absolute rest ; for although the outwardly visible pro- 

 cesses of development and growth stand still, the internal vital pro- 

 cesses, although retarded, never altogether cease. 



The annual periods of growth occasioned hy climatic changes, which are ren- 

 dered so noticeable by the falling of the leaves in the autumn, and the develop- 

 ment of new shoots and leaves in the spring, have stamped themselves so indelibly 

 upon the life of the trees and shrubs of the temperate zones, that, when culti- 

 vated in tropical lands where other plants are green throughout the year and 

 blossom and bear fruit, they continue to lose their leaves and pass for a short time 

 at least into a, stage of rest. The Oak and Beech have become so habituated to 

 this annual periodicity that they never depart from it ; other trees again gradually 

 accustom themselves to the new conditions, as the Cherry and Peach, for instance, 

 which in Ceylon have become evergreen trees. The Peach is reported to produce 

 flowers and fruit throughout the entire year ; while the Cherry, like many other 

 trees of the temperate zone, ceases altogether to bear flowers in tropical climates. 

 It is due to a similar habituation to an annual periodicity that in some cases it is 

 so difficult, or even altogether impossible, to induce plants by artificial culture to 

 flower out of season. The behaviour of different species also varies in this respect ; 

 in general, those flowers accommodate themselves best to forcing which, like the 

 Hyacinth, Crocus, Tulip, Syringa, and Oomus mas, naturally flower early. That 

 the internal vital processes are not promoted by artificial heat to the same extent 

 as growth in length, is at once perceptible from the abnormal appearance of many 

 forced plants whose leaves and flowers do not attain their full development (the 

 flowers of the Lily of the Valley, when forced artificially, develop even before the 

 leaves). 



Low temperature, especially frost, is often of advantage in the preparatory 

 vital processes during the period of rest ; this is made evident by the accelerated 

 transformation of the reserve material, and by the active growth in spring. 



Although to so many plants winter is the season of rest and cessation from 

 growth, other plants, e.g. certain Lichens and Mosses, seem to find in the warmer 

 days of winter the most favourable conditions of vegetation ; and in summer, on 

 the contrary, either do not grow at all or only very little. Similarly, many spring 

 plants attain their highest development, not in summer, but during the variable 

 weather of March and April, and, for the most part, they have entered upon their 

 rest period when the summer vegetation is just awakening. 



In countries where there are alternate rainy and dry periods, the latter generally 

 correspond to the winter period of vegetative rest. 



Dwation of Plant Life. — The life of a plant, during the whole of 

 its development, from its germination to its death, is dependent upon 

 external and internal conditions. In the case of the lower vegetable 



