1 8 LANG, Morphology of the Stock of Isoetes lacustris. 



phoric stele. Occasionally, however, equally large plants 

 are met with in which this secondary growth is almost 

 completely wanting. In such stocks the only secondary 

 growth is around the narrow base of the stem stele ; the 

 small amount of prismatic tissue formed there allows of 

 the widening rhizophoric stele fitting on to the stem stele 

 without dislocation of the cortical tissues. Plants of this 

 type are of great interest as bearing on the distinction of 

 the primary structure and on the part played by the 

 secondary meristem in the growth of the stock. 



Fig. 5. — Outline of a median section in the plane of the lobes 

 through a complete plant of Isoetes lacustris. The cambium and 

 .secondary prismatic tissue could not be indicated on this scale. 



Description in Text. 



In relation to this a word may be said as to the nature 

 of the continued cortical increase, which, as Fig. 5 shows, 

 carries the older leaves and roots outwards, providing 

 room for the more recently formed and functional leaves 

 and roots. This cortical extension has, since Hofmeister's 

 work, usually been described as due to the activity of the 

 secondary meristem. An alternative explanation, dealt 

 with in gi eater detail below, is to regard it as a continua- 



