120 BOTANY. 



One of the commonest forms is the field horse-tail (Equi- 

 setum arvense), a very abundant and widely distributed species. 

 It grows in low, moist ground, and is often found in great 

 abundance growing in the sand or gravel used as "ballast" for 

 railway tracks. 



The plant sends up branches of two kinds from a creeping 

 underground stem that may reach a length of a metre or more. 

 This stem (Fig. 72, (7) is distinctly jointed, bearing at each 

 joint a toothed sheath, best seen in the younger portions, as 

 they are apt to be destroyed in the older parts. Sometimes 

 attached to this are small tubers (o) which are much-short- 

 ened branches and under favorable circumstances give rise to 

 new stems. They have a hard, brown rind, and are composed 

 within mainly of a firm, white tissue, filled with starch. 



The surface of the stem is marked with furrows, and a sec- 

 tion across it shows that corresponding to these are as many 

 large air spaces that traverse the stem from joint to joint. 

 From the joints numerous roots, quite like those of the ferns, 

 arise. 



If the stem is dug up in the late fall or winter, numerous 

 short branches of a lighter color will be found growing from 

 the joints. These later grow up above ground into branches 

 of two sorts. Those produced first (Fig. 72, J.), in April or 

 May, are stouter than the others, and nearly destitute of chlo- 

 rophyll. They are usually twenty to thirty centimetres in 

 height, of a light reddish brown color, and, like all the stems, 

 distinctly jointed. The sheaths about the joints (L) are much 

 larger than in the others, and have from ten to twelve large 

 black teeth at the top. These sheaths are the leaves. At the 

 top of the branch the joints are very close together, and the 

 leaves of different form, and closely set so as to form a com- 

 pact cone (a;). 



A cross-section of the stem (D) shows much the same struc- 

 ture as the underground stem, but the number of air spaces is 

 larger, and in addition there is a large central cavity. The 



