i88 SECTIONAL ADDRESSES 



But other changes are visible in the procambial tissue almost immediately. 

 The outermost cells begin to vacuolate and diflFerentiate into protophloem 

 elements with thickening of vv^all and ultimate loss of contents, the change 

 being initiated from below upwards in continuity with the differentiated 

 sieve-tubes of the vascular strands below. Then another change is 

 noticed at the base of the leaf primordium ; on the inner (adaxial) side 

 of the procambial strand cells begin to vacuolate and differentiate into 

 lignified xylem elements, but even earlier the growth and division of the 

 elongated meristem cells in the centre of the strand has taken a new form. 

 These elongated cells at first expanded mainly in length, with the natural 

 result that they subsequently divided by a transverse wall. These cells 

 now grow more rapidly than can be allowed for in the longitudinal exten- 

 sion of the system as a whole. As a result they increase in size in the 

 radial and tangential directions and new cell divisions occur repeatedly in 

 which the new cell walls are tangential in the axis and parallel to the 

 flattened surfaces of the leaf primordium. These repeated tangential 

 divisions in elongated meristematic cells are characteristic of the cambium, 

 and their occurrence marks the change from procambium to cambium. 

 The new cells formed by such divisions lie in radial files which are visible 

 at a very early stage of shoot extension. Evidently, even during extension 

 growth of the shoot, the meristematic cells of the cambium are growing 

 more rapidly than the vacuolating dividing cells around them. The 

 vacuolating cells cease to divide altogether as the intercellular space 

 system around them fills permanently with air ; but the meristematic 

 cells of the cambium lie between the differentiating vascular elements, 

 from which supplies of sap still reach them along walls which are in close 

 contact, or between which small intercellular spaces are only just developing. 

 In the fully extended axis, therefore, the growth and division of the 

 cambium cells still continues, the new walls being mainly in the tangential 

 direction ; from the inner cells of the radial files thus formed, new 

 xylem elements are differentiated, from the outer cells new phloem 

 elements. 



The tree is the outcome of the maintenance of this radial growth in 

 the axis long after the leaf in which it was initiated has fallen, but it is 

 essential to realise that this radial increase by cambial activity is in direct 

 continuity with the normal meristematic growth of the shoot apex, and 

 is initiated always, at successively higher levels in the extending shoot apex, 

 at the base of each new leaf primordium. This fact is quite definite, 

 and whilst the protophloem on the outside of the procambial strand 

 always differentiates forwards into the shoot apex as an extension from 

 previously differentiated elements below, cambial activity and xylem 

 differentiation begin afresh at the base of each new leaf primordium and 

 extend from thence downwards as the growth of the vacuolating and 

 dividing tissue around them produces the axial extension we know as the 

 internode. In each developing internode, therefore, cambial activity and 

 xylem differentiation are most developed at the top of the internode, at 

 the point of leaf insertion, and from thence diminish in amount 

 downwards. 



So long as the internode is extending in length, the vascular elements 



