120 "SCOURING-RUSH," OR "HORSETAIL." 



merely to stiffen the nodes and to protect the basal grow- 

 ing regions of each internode. 



The stems are jointed, tubular, and ridged, the chlor- 

 ophyll appearing within the ridges, and the stomata 

 along their slopes. The stems also contain much silica 

 which makes them gritty. 



Surplus food is stored in the rhizome, and is used 

 partially or wholly as nourishment for the sporiferous 

 stalk early in the succeeding season. 



In the groups of sporangia on the sporophyll many 

 spores are borne. Each spore is covered as it develops 

 by a special outer coat which cracks at maturity in a 

 spiral fashion and so forms the elaters, not at all the 

 morphological equivalents of elaters of liverworts. As 

 the spore becomes dry the elaters straighten, and in 

 doing so jerk the spores about from place to place. 

 Becoming entangled, they may also hold several spores 

 together for a time. 



The spores produce gametophytes, each of which 

 bears but one kind of sex-organ, i.e. is dioecious. Since 

 elaters hold the spores in masses, one kind of sex-organ 

 is more likely to be formed in the vicinity of the other 

 than would be true if the spores were scattered singly. 1 



Equisetums were far more abundant and luxuriant for- 

 merly than now. Fossil remains show that during the 

 coal ages some genera of the order were often large trees 

 that composed a prominent part of the vegetation. These 

 highly specialized forms have ceased to exist and are 

 now represented only by their fossils and by their lowly 



1 If fresh spores may be had, it will be possible to study the gameto- 

 phytes and sex-organs. The spores germinate quite readily and must 

 be used soon after being gathered, else they lose their power to germinate. 



