K.— BOTANY 189 



differentiating from the cambium are pulled out during differentiation 

 and are characterised as protoxylem and protophloem. It has only recently 

 been realised that in many plants much, if not all, of this protoxylem 

 differentiates from cells cut off in regular radial rows from the cambium, 

 so that it is ' secondary ' in origin. After extension has ceased, the 

 vascular elements, metaxylem and metaphloem, differentiate, without 

 undergoing further extension, from the elongated cells cut off by tangen- 

 tial division from the cambium. 



This process of radial growth continues in the axis as long as the leaf 

 is active, being renewed in successive years when the leaf is evergreen ; 

 but when the leaf dies the process does not necessarily stop. Higher on 

 the axis are now developing new leaves ; new impulses to cambial activity 

 and vascular differentiation are travelling basipetally downwards through 

 the newly extending internodes, and these impulses do not cease to be 

 effective at the base of an internode. On the contrary, they continue 

 downwards into the fully extended internodes below, so that the cambial 

 activity and proto-vascular elements of the upper internode are in direct 

 and causal relation with the cambial activity and meta-vascular elements 

 of the internodes below, and, so long as new leaves are growing at the 

 apex, radial growth of the vascular tissues continues on the fully extended 

 axis below. It is the essential feature of the tree habit of growth that 

 this radial growth of the vascular system does not cease at the base 

 of the current year's shoot, but continues as a similar impetus to 

 new radial growth, from the base of the growing bud down over the 

 surface of the wood throughout the permanent woody axis of the 

 tree. 



Basipetal Cambial Activity from the Buds. — So long ago as 1862 Th. 

 Hartig pointed out, in the willow, that the new cambial activity on the 

 dormant woody twig began at the base of the buds and worked from 

 thence basipetally downwards, and that this original direction of growth 

 continued in a willow cutting even when the twig was inverted. The 

 significance of this observation does not appear to have been realised 

 at the time, and the fact was rediscovered recently and attention drawn 

 to it by several workers. Using ordinary anatomical methods, it is a very 

 tedious task to determine where cambial activity is renewed in the 

 spring, and it is not surprising that statements upon the subject are very 

 contradictory. During the last few years detailed studies of two species, 

 one hardwood and one softwood, by two Leeds workers, Mr. G. 

 Cockerham and Dr. W. Wight, have shown that in both the first inception 

 of cambial activity on the woody axis is to be found at the base of the 

 buds, and from thence cambial activity and vascular differentiation spread 

 basipetally downwards throughout the tree. Many thousands of sections 

 had to be examined to establish these facts. Fortunately in the present 

 year a new and simple method of following the renewal of cambial 

 activity has been found, which has rapidly extended the range of our 

 observations. 



In the resting condition the cambium cells on the surface of the old 

 wood seem to be relatively firm in texture. They are then very granular 

 in appearance with somewhat thick walls, and are bound firmly between 



