Ixxii. PRESIDENT'S ADDRESS. 



cold and wet, but where there is a recurring period of suspension 

 in the activity of growth, the annual rings are more clearly 

 defined. Sachs says " In tropical woody plants, when they 

 are several years old, the additions to the wood formed in each 

 successive year are not distinguishable on a transverse or longi- 

 tudinal section. The entire wood is homogeneous." During each 

 season of growth there is a marked separation between wood 

 formed in the spring, and wood added in the autumn. The part 

 formed in the early part of the year is called spring-wood, that in 

 the latter, autumn-wood. The explanation of the formation of 

 annual rings is not quite so simple and easy as is often supposed, 

 for we have not as yet sufficient data with regard to the rings of 

 growth in trees of different latitudes to allow us to speak with any 

 certainty upon questions of past climates. Many tropical plants 

 flower all the year round, and there ife no interruption to the 

 thickening of their stems ; others pass through a resting period 

 brought on by a long continuous period of drought. 



I conclude by quoting Sachs's remarks on the question of 

 dormant periods : " This periodic alternation of vegetative 

 activity and rest is in general so regulated that, for a given species 

 of plant, both occur at definite times of the year, leading to the 

 inference that the periodicity only depends upon the alternations 

 of the seasons, and therefore chiefly upon that of temperature and 

 moisture. Without wishing to deny the co-operation of these 

 factors, a closer consideration shows that this matter depends 

 chiefly upon changes which take place in the resting plant, 

 independently of external influences, or only indirectly affected 

 by them." Our knowledge of the structure of the tissues of fossil 

 plants, especially those of the Carboniferous Age, has in recent 

 years made considerable progress. Botanists recognise more and 

 more the importance of Paleeobotany, and fossil plants have ceased 

 to be regarded simply as aids, and of no great value to the 

 stratigraphical geologist. We may hope that a closer study of the 

 ancient floras of which the strata afford such numerous, 

 though fragmentary relics may make us better acquainted 



