1 



^^S SECONDARy CHANGES. 



tangential diameter, or breadth, of all cells taking part in the dilatation, is most 

 conspicuous, and follows obviously from what has been stated. In proportion, 

 however, to the increase in breadth, successive radial bipartitions take place, by 

 means of which the original breadth of the cells is approximately restored, and 

 the number of the cells in each tangential row increased in a corresponding degree. 

 These phenomena also take place in the endodermis of the stems, mentioned at 

 pp. 121 and 415, and of the roots, so long as it is not thrown off and thus excluded 

 from the growth. An increase in the average breadth of the individual cell no doubt 

 takes place, judging from estimates. It appears to rise rapidly to an approximately 

 constant value, and then to maintain this during the succeeding divisions, so that 

 cells of the same layer in a stem a foot thick are no broader than in one as thick as 

 one's finger, though they are of course more numerous in a corresponding degree. 

 The final, constant, average dimensions are relatively little in excess of those 

 existing originally at the beginning of growth in thickness ; they may be estimated 

 to amount to scarcely more than two or three times the latter. Accurate measure- 

 ments are still to be undertaken. Cell-division in directions other than the radial, 

 resulting in a multiplication of the concentric layers of parenchyma, is rare, at least 

 in the cortex of those woody plants which have been principally investigated, and 

 apart from the form.ation of periderms : those special cases in which it does occur 

 will be discussed below (Sect. 172); it remains to be investigated whether or not 

 such division also takes place in many fleshy roots. 



The structure of the cell-walls and of the contents, the periodic variation in the 

 amount of starch in the latter, &c., remain the same in the principal mass of the 

 parenchyma during the dilatational changes, in certain cases throughout Hfe, in 

 others for a time. Sooner or later, however, changes may occur, and in fact 

 {a) dilatational changes of the collenchymatous hypodermal layers (p. 404), and 

 (^) processes of secondary sclerosis. 



The coUerichymatous hypodermal layers of the cortex of stem and branches in 

 \voody plants always follow the dilatation uniformly for a time in their whole 

 circumference, while they maintain their original characteristic structure ; this process 

 often continues uniformly, so long as they are not thrown off by the formation 

 of bark ; the question whether, in many cases, their walls decrease somewhat in 

 thickness, as the extension proceeds, remains to be more accurately investigated. 

 In some cases, which might no doubt be multiplied by further observations, namely, 

 in Tilia, Acer striatum, and -^sculus, other conditions prevail. At certain points the 

 collenchymatous cells show a considerably greater growth in breadth than in the 

 intermediate tracts, and all their walls, both those originally existing and the portions 

 added by subsequent growth, decrease considerably in thickness. They permanently 

 assume the appearance of thin cellulose-w-alls, and are thus sharply distinguished 

 from the neighbouring thick, brilliant, collenchymatous walls. With this is con- 

 nected, at least in the case of Acer striatum, a diminution in the amount of 

 chlorophyll, which is apparent to the naked eye. The process begins in a few small 

 portions of each transverse section, spreads laterally from these points, and involves 

 new regions lying between the first. The thick-walled collenchyma in this w-ay 

 becomes subdivided into constantly smaller portions, lying isolated among the altered 

 tissue, and at last disappear?, as these portions also become involved in the process 



