igo SECTIONAL ADDRESSES 



the surfaces of the wood and bast, so that the bark will not ' slip.' The 

 first sign of renewal of cambial activity is a change in the consistency of 

 the cambial cells. Their contents become much more transparent and 

 apparently semi-fluid, and the bark now ' slips ' easily upon the surface 

 of the wood, separating from it at this plastic cambium layer. But if the 

 bark is peeled off a little later, after a few divisions in the cambial cells 

 have taken place, then the separation still takes place at the cambium 

 layer, and as most of the newly formed cells have usually been cut off to the 

 inside of the cambium, these now lie in a thin film on the firm surface 

 of the old wood. These new cells have thin walls and fluid contents and 

 can readily be stripped off the surface of the old wood. In this manner 

 long strips of tissue newly formed from the cambium, in which proto- 

 plasmic streaming has frequently been seen and in which early stages 

 of vascular differentiation are readily visible, can be stripped from the 

 surface of the old wood with the greatest ease. By the use of this method 

 it has been possible to follow the resumption of cambial activity in a 

 number of species of both hardwoods and softwoods. 



The results will be presented in detail elsewhere, but the general 

 result is a complete confirmation of the conclusion that the renewal of 

 cambial activity upon the surface of the old wood depends upon the 

 commencement of growth in the buds. Such cambial activity always 

 begins beneath the buds and spreads from thence basipetally downwards. 

 In the softwoods the basipetal spread of cambial activity is extremely 

 rapid. In some hardwoods, as oak, ash, sweet chestnut and elm, it is 

 also extremely rapid, but in others, as in sycamore and horse chestnut 

 and many of the Rosacea;, the downward spread of cambial activity is 

 much slower. In birch, beech and alder again, the buds have burst 

 and the leaves emerged before there is any sign of cambial activity 

 spreading down the twigs ; but as the extension growth begins in the new 

 shoots, cambial activity appears on the woody shoots beneath the buds 

 and spreads from thence relatively rapidly down the tree. The varied 

 details of this process have proved exceedingly interesting, and there is no 

 doubt that the new method has much to tell us of the characteristics of 

 radial growth in different trees . The ring-porous type of wood characteristic 

 of oak, ash and elm is evidently connected with the rapid and early spread 

 of cambial activity and vascular differentiation down the axis, whilst the 

 diffuse-porous type of wood of beech links with its later basipetal spread 

 of cambial activity. At the moment however, we must be content to empha- 

 sise the significance of the general conclusion that the resumption of 

 radial growth of the trees is almost completely dependent upon the 

 commencement of growth in the buds. With this clue to their interpre- 

 tation, the problems of form, structure and function presented by the 

 tree are seen in quite a new light. 



I, Form. 



We can only indicate, by discussing one or two examples, how tree 

 form is dominated by this causal link between bud development and 

 radial growth. One familiar horticultural operation demonstrates the 



