T20 



MORPHOLOGV OF TISSUES. 



of the flat lateral portion (Fig. 99,,/;/') proceeds from the sections in front which 

 face the margin of the shoot ; and this tissue is only one cell-layer in thickness, 

 no division taking place in it parallel to the surface of the shoot. All the divisions 

 in these marginal sections of the segment are, on the contrary, at right angles to 

 the surface of the shoot, and are produced by the marginal section first of all 

 breaking up into two cells lying close to one another (cf. Fig. 100, A, 0), each 



Fig. 99.— Apical region of a shoot oi Metzge7-ia fiircata in the act of dichotomous branching, looked at from the surface (after 

 Kny). The shoots consist of a single layer of cells (ff), which is however penetrated by a mid-rib n n', three to six layers iis 

 thickness. 



Fig. 100. — Diagrammatic representation of the segmentation of the apical cell, and of the first divisions in the segment of 

 Metzgei-ia fiircata (after Kny). A apex seen from the surface ; B the same in vertical longitudinal section ; C an apex in the act 

 of dichotomous branching ; a new apical cell is formed in the third-youngest segment. 



of which then forms several shorter cells by repeated bipartition, and these may 

 again undergo further division according to the activity of the growth. In general 

 the first divisions only of the segment are constant ; the further course of cell- 

 multiplication is, according to the minute investigations of Kny, subject to many 

 deviations. Since the tissue which is produced from the marginal sections assumes 

 a prominent position during growth, it results that the apical cell lies, with the 

 youngest segments, in a depression of the outline of the shoot ; and thus we have 

 here a simple example of the depression of the punctum vegetationis in the tissue 

 which grows more luxuriantly' around it, such as often occurs to a much greater 

 extent in Fucaceae, Ferns, and Phanerogams. The differentiation of the tissue out 



