SECONDARY THICKENING. NORMAL DICOTYLEDONS. 469 



radial wall in question. (Comp. Fig. 202 m.) In the first cell thus formed, further 

 transverse divisions may then ensue. In the frequent case where a medullary ray, 

 originating as described, increases in height in the successive zones of secondary 

 growth, both as regards its absolute size and the number of its cells, the various 

 possible modes in which new cells may be added may easily be perceived, but the 

 actual process has not been established with certainty. Further, it is not clear 

 whether a small medullary ray may not be formed by repeated transverse division of 

 an entire elongated cambial cell, or even of several one above another. 



Sect. 137. As follows from their mode of development, the secondary elements 

 of the wood and bast are always at first arranged in radial rows, if we except many 

 groups of sieve-tubes. A growth in thickness of the masses of tissue lying inside the 

 actual zone of secondary growth, such as would result in a displacement of the radial 

 rows, takes place in the case of the wood exclusively during the development of the 

 innermost layers, which are affected by it in the way of displacement or tangential 

 extension; at a later period nothing of the kind occurs; in the case of the bast the 

 conditions are no doubt different, owing to the continual widening of the zone of 

 secondary growth, but the displacements following from this, which are to be described 

 below, affect only the old external zones to any considerable degree. 



The secondary elements must therefore maintain their original radial arrange- 

 ment : 



(i) When the form and length which they had in the cambial stage undergoes 

 little or no change on their definitive development. 



(2) When, although they become larger than the cambial cells, they maintain a 

 form similar to the latter, and when, in particular, they have terminal surfaces in- 

 clined only towards the radial plane, where they abut on and penetrate between 

 one another. 



The first case occurs almost without exception in the medullary rays of normal 

 structure (for the exceptions in the case of Atragene, Casuarina, &c. see below), in 

 most parenchymatous masses of the wood, and in short tracheides, e. g. those of 

 Cytisus Laburnum, &c. It is true that the height and breadth here frequently in- 

 crease somewhat, even in the medullary rays \ on the transition from the cambium 

 to the definitive condition, but only in a slight degree, which does not alter the general 

 grouping. 



The second case applies to the elongated elements (fibrous cells, woody fibres, 

 tracheides) of the wood of many, and the bast of most plants which form wood. 

 Even, however, in cases belonging to this category, the longitudinal growth of the 

 parts passing out of the cambial condition is trifling, as will be shown in the following 

 paragraphs. 



The conditions in question are present, almost without exception, in all parts of 

 the wood and bast of the Coniferse ; in the Dicotyledons they are tolerably general 

 in the soft bast, with the exception, however, of those cases which are characterised by 

 irregular groups of sieve-tubes. 



The sclerenchymatous fibres of the bast maintain their radial arrangement, for 

 example, in Carpinus, Corylus, Ostrya, Liriodendron, and Magnolia acuminata and 



Compare Hofmeisler, Pflanzenzelle, p. 164. 



