174 THE BOOK OF NATURE STUDY 



another interesting lesson. Some trees do not form seed re- 

 gularly. The Common Elm is a case in point. This tree was 

 introduced into England by the Romans, and does not seed nearly 

 as regularly as the Wych Elm, which is indigenous. The common 

 elm is propagated by suckers, not by seed. It differs from the 

 wych elm in its greater height and finer grained wood. Its 

 seed, when formed, is nearer the notched end of the fruit. Elms 

 would not hold their own in woods, but would probably soon 

 be ousted by other trees ; they are found in parks or in avenues, 

 and are often planted in hedgerows. 



Thus, as the seasons come round, they bring with them a 

 regular succession of changes in the life of a plant. What is 

 the cause of these changes ? Is the plant merely responding to 

 the influence of external conditions, or do plants inherit a ten- 

 dency to change with each season ? There are many facts 

 which seem to support either view. There is no doubt that 

 increase of temperature does shorten the resting period, and 

 that prolonged cold does lengthen it. The mild weather of 

 November and December 1907 led many plants to flower some 

 weeks earlier than usual, and to keep their leaves much longer 

 than usual ; and a cold spring may retard vegetation for at 

 least a month. A comparison of records of early flowering, if 

 kept for some years, would give surprising results. This is not 

 all. There are cases on record in which plants have adopted 

 a different habit when exposed for a sufficient length of time 

 to a different environment. Schimper mentions certain de- 

 ciduous trees which, growing on the flanks of the volcano of 

 Gedeh in Java, where the climate is temperate and constantly 

 humid, have adopted the habit of the evergreen. He says 

 that the buds expand not simultaneously but successively, 

 so that an individual plant bears at the same time spring, summer, 

 autumn, and winter shoots. In the course of a few years the 

 resting period of a plant disappears in a warm, humid climate. 

 In the cultivation of varieties new plants have been produced 

 in which the resting period differs from that of the parent stock. 

 Facts of this kind do show the immense influence exerted by 

 external conditions, as would be expected. At the same time, 

 environment is not the only factor to be considered. The 



