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 



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Fig. 28. — Isoetes lacusiris 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; 



