80 STRUCTURAL BOTANY 



(as in our type, shown in Fig. 38) there are special fertile 

 stems which only bear the cones, but do not branch, 

 and are not green. In others, the cones are borne on 

 the ordinary green vegetative stems. Underground the 

 plant has a much-branched rhizome, which penetrates 

 to a great depth in the soil, and makes these plants most 

 obstinate weeds. If such a species as E. arvense or E. 

 maximum has once established itself in garden ground, 

 it is almost impossible to get it out again, for its rhizome 

 goes too deep to be easily dug up, and is perpetually 

 giving rise to new shoots. 



Equisetum, as we shall ficd, resembles other Vascular 

 Cryptogams in having a sharply marked alternation of 

 generations. The plant, as we see it, is the asexual 

 sporophyte, and with this we will begin. 



I. EXTERNAL CHARACTERS OF THE SPOROPHYTE 



A. VEGETATIVE ORGANS 



The general habit of the commonest British species, 

 E. arvcnse, is well shown in Fig. 38, but only some of 

 the upper branches of the rhizome are represented. We 

 must picture to ourselves the main part of the rhizome 

 deep down in the soil, perhaps three feet below the 

 surface, sending up branches which alone are visible in 

 the figure. The characteristic leaf-sheaths are obvious 

 on all the stems whether above or below the ground; 

 on the older parts of the rhizome, however, they often 

 wither away. Each sheath consists of a whorl of 

 coherent leaves, the free parts of which are only repre- 

 sented by the teeth at the top of the sheath. 



The rhizome bears numerous slender adventitious 



