PHYSIOLOGY 261 



They are then first enabled to fully exercise their special function. 

 To this end the cavities of certain cells usually become more or less 

 fused, and the cell walls thickened, often in a peculiar and character- 

 istic manner (p. 63). 



Periodicity in Development and Duration of Life 



The periodically recurring changes in the determinative external 

 influences, especially in light and temperature, occasioned by the 

 alternations of day and night and of the seasons, cause corresponding 

 periodical variations in the growth of plants. These variations do 

 not follow passively every change in the condition of the external 

 influences. On the contrary, the internal vital processes of plants so 

 accommodate themselves to a regular periodicity that they continue 

 for a time their customary mode of growth, independently of any 

 external change. The nightly increase of growth, which is especially 

 noticeable after midnight in the curve of growth, and the retardation 

 of growth, specially marked after mid-day, will continue to be 

 exhibited 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 an after effect of the usual rhythm which 

 suggests a comparison with the faculty of memory, and will be further 

 considered below. According to SEMON, the usual periodicity may 

 even be transmitted to the descendants of a plant. 



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 

 processes, although retarded, never altogether cease. 



The ANNUAL PERIODS of growth occasioned by climatic changes, which are 

 rendered 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 cultivated 

 in tropical lands where other plants are green and blossom and bear fruit through- 

 out the year, they continue to lose their leaves and pass for a short time at least 

 into a stage of rest. The Oak, Beech, Apple, and Pear retain their resting period 

 in the sub -tropical climate of Madeira, while, under uniformly favourable 

 conditions in the mountain regions of Java, the periodicity may be disturbed in 

 particular individuals. This even occurs in the several brandies of the same tree 

 which may then bear leafy and leafless boughs at the same time (Oaks. Magnolias, 

 Fruit and Almond trees, together with some endemic species). Other trees again 

 gradually accustom themselves to the new conditions, as the Peach, for instance, 

 which in Ceylon has become an evergreen tree. 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. 



