420 



LECTURE XXVI. 



The growing-points, as well as the young organs which are still in an embryonic 

 state, constitute a mass which is homogeneous in itself; the cells appearing as mere 

 chambers, the growth of which depends upon the growth of the growing-point as 

 a whole. 



It is not quite correct to say, as is often done, that the cells in the growing- 

 points and in the young organs — in embryonic tissue generally — are perfectly 

 homogeneous among themselves. On the contrary, differences in form make 

 themselves evident even close to the apex of the growing-point, as well as in its 

 very young outgrowths ; and in the growing-points of roots especially, it is possible 

 to detect with certainty, even close behind the apex, those cells which will subse- 

 quently become developed as segments of vessels, or as endodermis, &c. (Fig. 257). 

 In the same manner, the external layer of the growing-point and every embryonic 

 organ is easily recognised as what will be the future epidermis. According to the 

 various form of plant in each case, however, this commencing differentiation of the 

 tissues in the growing-point is very different. The doctrine of the three so-called 

 histogenic layers, according to which, besides the future epidermis, a so-called 



Fig. 256.— The growing-point of a shoot (winter-bud) of the Pine. 5 apex of the growing-point ; 

 l> h youngest leaf-rudiments; rr young cortex, and mni pith of the future shoot-axis (highly 

 magnified). The whole consists of embryonic tissue, of very small cells with relatively large nuclei. 



periblem (young cortex) and a plerome are sketched out in the growing-point itself, 

 cannot therefore be applied universally; although the tissue differentiations referred to 

 often do occur, particularly in the growing-points of roots. 



(2) The second phase of growth, the so-called condition of elongation, is 

 introduced -by the cells at the base of the growing-point beginning to grow more 

 rapidly, and to approach their definitive development. The following are the chief 

 points to be noted here : — 



By means of the elongation, the segments of the shoot-axis, as well as the 

 leaves and the corresponding parts of the roots, attain their definitive size and their 

 permanent external form. 



This process may continue for a longer or shorter time ; and, accordingly, the 

 elongating piece of shoot-axis or root-fibre is longer or shorter, and attains during the 

 elongation a more or less considerable breadth and thickness. The length of the 

 elongating portion in a root-fibre is about 3-10 millimetres behind the growing-point, 



