4S THE HORSETAILS. 



branch again, as they do in the wood horsetaiL In 

 these forms the branches have no central hollow, and 

 many of them are more than three-angled. 



The fertile fronds seem little inclined to change with 

 the changes of the sterile one. Occasionally, however, 

 small catkins appear on the tips of sterile stems in 

 early summer, forming the variety campestre. This is 

 supposed to be due to a late spring frost cutting down 

 the regular fertile fronds, and thus throwing a fruiting 

 tendency into the sterile stems. 



The . main rootstock of this species is perennial, 

 branching, and creeps extensively a foot or more below 

 the surface of the earth. At short intervals secondary 

 rootstocks are given off, and these go directly to the 

 surface, there giving rise to both fertile and sterile fronds. 

 The rootstocks are in all respects like the stems except 

 that they lack the central hollow. The internodes of the 

 secondary rootstocks are often heavily clothed with short, 

 tawny felt, and at the nodes, as in the main rootstock, 

 the usual sheaths occur. From the base of the buds in the 

 axils of these sheaths the roots are produced. When the 

 buds develop they produce rootstocks, though if exposed 

 to the air they may take on the form and function of stems. 

 They often remain dormant for an indefinite period, but 

 will grow at once if needed in the economy of the plant. 



The secondary rootstocks often bear at the nodes 

 small rounded tubers as large as peas. These have the 

 same structure as the rootstock, and are probably in the 

 nature of arrested branches. More than a dozen of 

 these tubers have been found on a twelve-inch section 

 of secondary rootstock. It has been conjectured that 

 these tubers act as storehouses of food upon which the 

 plant can draw when the fronds are developing. 



