THE COXE OF EQUISETUM 327 



The stems are generally of two forms.* There are green aerial 

 stems above ground, imbranched in some species, but quite 

 bushy in others by the development of circles of side branches 

 at the joints (Fig. 284, B). The aerial stems arise from creeping 

 underground steins, or rootstocks, which have the same jointed 

 structure and sheaths of degenerate leaves, but are not green 

 and often not hollow. The underground stems live from year 



O t/ 



to year and grow rapidly through the soil, frequently estab- 

 lishing large beds of horsetails, as, for example, along railroad 

 tracks and the margins of sandy pools and ponds. 



The stem has large, central air cavities, running from joint to 

 joint, and also a number of smaller air canals, alternating with 

 the fibre-vascular bundles (Fig. 284, F, a). It is strengthened 

 by thick-walled cells, forming a rigid tissue (Fig. 284, F, r) under 

 the epidermis, and is consequently well protected from the dan- 

 ger of drying up. These peculiarities, together with the reduced 

 leaf surface, are characters which the horsetails have in common 

 with many desert plants (xerophytes), and they permit them to 

 live when necessary under very severe drought conditions. 



The fructification of Equisetum is a cone (Fig. 284, A) devel- 

 oped at the tip of the stem, and it is composed of scale-like 

 spore leaves (sporophylls), which fit closely together and develop 

 spores in sporangia upon their under surfaces (Fig. 284, E). 

 These cones are generally found on ordinary green stems. How- 



*J V I/' C_7 



ever, in some species, as E. arvense, the stems which first appear 

 above ground are pale in color and are devoted entirely to the 

 development of the cones and die after the spores are shed, while 

 the green vegetative stems appear later. 



317. The cone of Equisetum. A cone, or strobilus, is a com- 

 pact group of spore leaves (sporophylls) distributed around the 

 tip of a stem and distinct from the rest of the plant. It takes 

 its compact form because the sporophylls are closely set together 

 and frequently so much modified that their structure is not 

 apparent at a glance. Each sporophyll in Equisetum consists 

 of a short stalk attached to the side of the stem and bearing an 



