i86 SECTIONAL ADDRESSES 



and forestry, its contribution to make in the service of mankind. He 

 laid stress upon this because he felt that a recognition of its practical 

 significance would vitalise botanical teaching, which was ' inclined to 

 elaborate the minute detail of a part at the expense of its relation to the 

 whole organism, and discuss the technique of a function more in the 

 light of an illustration of certain chemical and physical changes than as a 

 vital phenomenon of importance to the plant and its surroundings.' 

 This tendency, Bayley Balfour argued, must be counteracted ' if botany 

 is in the future to be aught else than an academic study, as it was of 

 old an elegant accomplishment.' He sees the roots of the trouble in the 

 failure of botanists, so far, ' to see the lines through which the subject 

 touches the national life.' Bayley Balfour would possibly, therefore, 

 have felt some sympathy with this effort to show that a study of the 

 growing tree throws fresh light upon its form, structure and vital functions, 

 and gives new meaning to the practices of the forester and horticulturist, 

 whilst the details of structure which attract the attention of the worker in 

 wood are also seen in new perspective. 



The Habit of Growth of the Tree. 



The tree is characterised essentially by prolonged vegetative growth 

 and delayed reproduction. Green leaves add to the substance of a 

 plant by their activity, whilst flower and fruit production exhaust it, so 

 that during this prolonged period of vegetative activity the tree gains 

 annually in substance. It is a further characteristic of the tree that each 

 growing season sees this substance added as an increment of radial growth 

 upon a woody branch system which increases no more in length. Each 

 year also, emerging from the buds in which they lie concealed during 

 the dormant season, the growing points of the shoot form new extension 

 shoots bearing new leaves, although after flowering commences some of 

 these growing points also form flowers. 



At first sight the extension growth of the shoots from the buds, and 

 the formation of wood and bast on all the rest of the woody axis, seem 

 two independent processes, but recent studies in the Leeds laboratories 

 have convinced me that the key to the interpretation of the behaviour of 

 the growing tree is to be found in the fact that these two processes are 

 inseparably and causally connected. This statement must first be justified , 

 and then it is hoped to show that, regarded from this angle, problems of 

 form, structure and function connected with the growing tree reveal an 

 entirely new significance. 



Apical Growth and Radial Growth. — Two great classes of plants, the 

 Dicotyledons and the Gymnosperms, are characterised, by growth pro- 

 cesses which continue to thicken the axis of the shoot after it has extended 

 in length. In these two groups are found the two characteristic tree 

 groups, the hardwoods in the Dicotyledons, the softw.oods in the Gymno- 

 sperms. To each great group also belong plants which are not trees, 

 the Dicotyledons in particular being predominantly herbaceous, but 

 throughout both groups the axis increases in thickness, after it is first 

 formed, by the continued growth of an internal cylinder of cells known 



