io6 SECONDARY CHANGES. 



depends'. In addition to these changes, the growing epidermal cells, as ihey 

 become larger in the direction of the circumference, i. e. broader, divide successively 

 by walls which stand at right angles to their transverse diameter, and to the surface, 

 and abut on the inner surface of the original wall. This successive multiplication 

 of the epidermal cells takes place in such a degree, that the average breadth and 

 general form of the individual cells remain approximately the same, or only undergo 

 inconsiderable changes. The epidermal cells of the stem of Acer striatum when 

 the latter is 200™™ thick, are, for example, scarcely twice as broad as those of a 

 shoot one year old, and 5™™ in diameter. 



Sect. 171. Parencliynia forms the main mass of the primary external cortex, 

 the medullary rays of various degrees in the bast-layer, and the parenchymatous 

 groups in the bundles of the latter. Until a zone of cortex is thrown off by 

 the formation of bark, which may take place sooner or later, but in many instances 

 never occurs at all, and which will be discussed below, the parenchyma follows the 

 cambiogenetic secondary growth by corresponding growth of its own, in all the 

 parts in which it exists. The parenchymatous mantle of the external cortex 

 increases successively in width, while the medullary rays of the bast, and the 

 parenchymatous elements of the bundles, increase in breadth in the centrifugal 

 direction (Fig. 214, p. 528). The several portions of the bast do not always 

 participate in the same degree in this phenomenon, which may shortly be termed 

 Dilatation of the parenchyma. If attention be directed to cases of extreme 

 difference, it is found that in the one case dilatation of the entire parenchyma 

 of the bast takes place in an approximately uniform proportion, as each annular 

 zone becomes shifted outwards. In all the radial bands, and thus most clearly 

 in the medullary rays of every degree, the parenchymatous cells increase uniformly, 

 and quite gradually in breadth, in the centrifugal direction. The intermediate non- 

 equivalent tissues, which do not grow with them, especially sieve-tubes and bast-fibres, 

 thus become uniformly removed one from another, and the more so the further 

 they are from the cambium ; as in Salix fragilis and allies, Punica, Rhamnus Fran- 

 gula ■■', Spiraea ulmifolia, Pyrus communis, and .fflsculus. In the other extreme case 

 the dilatation is unequal in the various radial bands of the transverse section; 

 it amounts to little or nothing in the bundles, and is most active, either in all 

 the parenchymatous rays, or in some of them. Between the lateral limits of these 

 dilated rays the arrangement, and lateral distance from one another of all elements of 

 the tissues, remains approximately the same. This behaviour occurs, firstly, in a 

 number of stems, which are constructed on the type described at p. 453, and the 

 large medullary rays of which are broad and multiseriate ; e. g. stems of Meni- 

 spermum, Aristolochia, and Piperacese ; here the dilatation is brought about, at least 

 to the greatest extent, by the large medullary rays, and, indeed, the latter all take an 

 approximately uniform share in the process, in their entire height. The strands of 

 bast therefore remain similar in form and arrangement to the phloem portions of 

 the original vascular bundles, from the further development of which they have 

 arisen. They are not, however, wholly without share in the dilatation, as a slight 



' For details compare Botau. Zeitg. 1S71, p. 605, &c. 

 ' Compare Berg, Atlas, Taf. 39, 40. 



