410 



VA SC ULA R CR YP TO GA MS. 



pair at right angles but obliquely, an arrangement which is often clearly seen on 

 old shoots of -S*. Kraussiana. 



The Apical Growth of the stem takes place, in Isoetes, Selaginella, and Psilotum, 

 by means of an apical cell. That of hoetcs lacustris is, according to Hofmeister, 

 two-edged when the stem has two furrows; in the species with three furrows it 

 is a three-sided pyramid. In young plants the leaves stand accordingly in the first 

 case in two, in the second case in three rows ; but later the phyllotaxis becomes 

 more complicated and spiral, indicating perhaps that in the older stem the primary 

 walls of the segments are arranged in regular succession, in the same manner as 

 in those Hepaticse which have a three-sided apical cell and a complicated phyllo- 

 taxis. In those species of Selaginella which have the leaves in four rows, the 

 apical cell of the stem is, according to Pfeffer, two-edged (Fig. 308, A, B). The 

 two rows of segments here form an elevated vegetative cone, at the base of 

 which the rudiments of the leaves first appear at the height of the fourth or fifth 



Pig. 30S.— Apex of the stem of Selaginella Martensii (after Pfeffer) ; A longitudinal section of the end of the stem 

 with the first rudiment of the leaves ; R apex of the stem seen from above ; C dichotomy of the apical cell seen from the 

 side ; D the same seen from below. The primary walls of the segments are denoted by darker lines ; the segments 

 themselves are numbered with Roman figures. 



segment. The two edges of the apical cell are directed upwards and downwards 

 (on the obhquely ascending shoot). The relationship of the leaves to the seg- 

 ments has not yet been entirely made out. The tw^o leaves of each pair arise ob- 

 liquely ; one above, the other below, and alternately right and left ; where the pairs 

 cross obliquely, each embraces about a fourth of the circumference of the stem. 

 Divisions then take place which are directed obliquely upwards and downwards, 

 and a row^ of apical cells is thus formed, by means of which the growth of the leaf 

 is continued (Fig. 308, A). The dichotomy of the shoot is caused by a second 

 two-edged apical cell being formed from the youngest segment (Fig. 308, C, D). 

 The two shoots which are thus formed grow right and left of the previous direction 

 of growth, and all the successive dichotomies take place in one and the same plane. 



In Psilotum triquetrum the root-like underground shoots have been investigated 

 by Nageli and Leitgeb in relation to their apical growth. They found a small 

 three-sided apical cell, the divisions of which however advance (as in Polytrichum 



