132 



EXTERNAL CONFORMATION OF PLANTS. 



Phanerogams, are related to the axis or stem from which they are derived in the 

 manner described in the following paragraphs. 



(i) The Leaves always originate below the groiving apex of the stem as lateral 

 outgrowths, either singly or several at the same height, i. e. at an equal distance 



Fig. 105. — Longitudinal section through the apical region of three primarj- shoots of Charafrag-ilis; t the apical cell, in which 

 segments are fornied b3' septa ; each segment being further divided by a curved septum into a lower cell no further divisible 

 wliich developes into an internode^'^'^'", and an upper cell which produces a node ;« ?«' and the leaves. Each node-cell 

 produces a whorl of leaves of different ages. (For a more exact description, see Book II. CharaceK.) 



Fig. 106.— Longitudinal section through the apical region of a stem of 

 Fontinalis antipyretica, a Moss growing in water (after Leitgeb) ; -z/ the 

 apical cell of the shoot, producing three rows of segments which are at first 

 oblique and afterwards placed transverselj' (distinguished by a stronger out- 

 line). Each segment is first of all broken up by the division a into an inner 

 and an outer cell ; the former produces a part of the inner tissue of the 

 stem, the latter the cortex of the stem and a leaf. Leaf-forming shoots 

 arise beneath certain leaves, a triangular apical cell z being formed from an 

 outer cell of the segment, which then, like v, produces three rows of seg- 

 ments; and each segment here also forms a leaf (A more exact descrip- 

 tion in Book II, under Mosses.) 



from the apex ; in the latter case 

 they form a whorl the single leaves 

 of which may differ in age, as in 

 Chara and Salvinia, and in the 

 leaf-whorls of many flowers. 



(2) So long as the punctum 

 vegetationis 0/ the shoot grows iji a 

 straight line at the apex, the por- 

 tion of the shoot ivhich produces 

 leaves lengthens, and the leaves 

 arise in acropetal order ; i. e. the 

 nearer the leaves are to the 

 apex, the younger they are ; in 

 this case leaves are never pro- 

 duced at a greater distance from 

 the apex than those already in 

 existence. It is only when, as 

 not unfrequently happens with 

 the flowers of Phanerogams, the 

 growth in length of the shoot 



