48 VASCULAR CRYPTOGAMS 



at both ends, where they are provided with two tufts of very long cilia ; 

 in some species they are remarkably large. Their period of ' swarming ' 

 does not last more than about five minutes. 



The stem of Isoetes is distinguished by its extraordinarily restricted 



FIG. 28. Isoetes lacustris L. (natural size). 



growth in length, and the complete absence of branching, as well as by 

 a remarkable secondary increase in thickness. It is completely covered 

 by the bases of the leaves, leaving no part exposed. Its upper portion 

 has the form of a shallow funnel, with the apex depressed in its centre. 



