4^4 SECOND A RV CHANGES. 



row to divide once into two, it is necessary that as many new elements should be 

 formed in the radial direction as are already present on the radius of the cambial 

 ring. ' An equal increase in the radial and tangential directions, so that two outer 

 cells would correspond to every inner cell, could only take place if the diameter 

 of the cell were equal to the radius' (i.e. only in the innermost* layer of cells of 

 a stem assumed to be destitute of pith). ' When the radius of the cambial ring 

 has the length of 50, or 100, or 1000 wood-cells the radial rows must be pro- 

 longed by 50, or 100, or 1000 cells in order to be doubled once.' The former of 

 the above assumptions holds good exactly for the boundary of the cambial zone 

 towards the wood ; the second only applies to particular cases. In other cases, in 

 which a successive increase of size of the secondary elements goes on, the relation is 

 still less favourable to the multiplication of the radial rows. In fact, according to 

 these considerations, the radial divisions in the initial layer must take place rarely in 

 comparison to the tangential ones, and indeed the former are found to appear here and 

 there in individual cells, in the course of successive secondary growth, without any 

 demonstrable order of succession. 



In the medullary rays the course of the secondary growth and of the divisions is 

 in general essentially similar, yet at least in the most frequent case, to be described 

 below, of radially elongated, parenchymatous elements of the medullary ray, it may 

 be simpler as regards the divisions, the cells following the growth of the wood- 

 strands for a longer time by radial extension, and the divisions happening more 

 rarely than in the strands ; they produce on the one side a new initial-cell, and on the 

 other a new tissue-cell directly, and without any further previous divisions. 



In addition to the divisions by vertical longitudinal walls, which alone have 

 hitherto been regarded, transverse divisions occur in the formation of the secondary 

 parenchyma, and no doubt oblique ones also in the origination of small medullary 

 rays. These can only be discussed below, after describing the conditions of form of 

 the cambial cells. 



If we wish to designate by the name camhitwi a zone strictly distinguished from 

 wood and bast, it consists, in accordance with what has already been said, of two, or 

 rather of three, different layers of cells, namely (i) the single initial layer, and 

 (2) the tissue mother-cells, including (<•?) those of the wood-side, and (b) those of the 

 bast-side. The contingent modifications in the case of the medullary rays need not 

 be repeated here. On the two layers of mother-cells border the products of their 

 division, which already belong to the wood or to the bast. As the definitive forma- 

 tion of these requires some time, and they must at their first origin be similar to their 

 mother-cells, a sharp distinction between them and the cambial zone is usually very 

 difficult in practice, even in the condition of winter's rest; in descriptions they are 

 therefore usually comprehended under the term cambium. We may distinguish them 

 from the true cambium as young wood or young bast ; in cases, however, where this 

 distinction is not practicable, or is a matter of indifference, while on the other hand a 

 distinction is required between the mature wood and bast and the collection of zones 

 just described, the latter may be included under the general name of the zone of young 

 secondary groivth, or the young secojidary growth, the term young secondary growth 

 being understood as opposed to the developed secondary growth, consisting of wood 

 and bast. 



