412 



BOTANY 



Class II 



Equisetinae (Horse-tails) 



The Equisetinae, which form an entirely independent class, include 

 only the one genus Equisetum, comprising 25 species, found widely 

 distributed over the whole world. Developed partly as land-, partly 

 as swamp-plants, they may always be distinguished by the character- 

 istic structure and habit of the asexual generation. They have a 

 branching, underground rhizome on which arise erect, aerial haulms, 



usually of annual growth. The 

 rhizome of the common Horse-tail, 

 Equisetum arvense, develops also short 

 tuber-like branches which function 

 as reservoirs of reserve material and 

 hibernating organs (Fig. 347, 2, a). 

 The aerial haulms remain either 

 simple, or they give rise to branch 

 whorls, and these in turn to whorls 

 of a higher order. Each axis con- 

 sists of a series of elongated inter- 

 nodes ; externally, it is 'channelled 

 by longitudinal furrows, while in- 

 ternally it is traversed by a central 

 air-passage and by a number of 

 smaller peripheral passages, valle- 

 cular canals, one opposite each of 

 "vaUecuiai' * ne furrows (Fig. 346). Between 



rows of stoma ta. (x 11.) 



a line with a surface ridge. 



Fig. 346. — Equisetum arvense. Transverse sec- 

 tion through the stem, m, Lysigenic medul- 

 lary cavity ; e, endodermis ; cl, carinal canals 

 in the bicollateral bundles ; vl, 



cavities; ftp, scierenchymatous strands in the central and vallecular canals, 

 the furrows and ridges; eft tissue of the and alternating with the latter, is 



primary cortex containing chlorophyll ; st, . . . °. .., . ' 



a circle ol bicollateral vascular 

 bundles, each of which is thus in 

 Each vascular bundle is also traversed 

 by a longitudinal water-passage, the carinal canal. 



The leaves of the Equisetinae, both in their structure and in the 

 manner of their arrangement on the stem, are also characteristically 

 developed. At each node is borne a whorl of scale leaves pointed at 

 the tips, and united below into a sheath closely enveloping the base 

 of each internode. The leaves of the successive whorls alternate 

 regularly with each other, and as each leaf is in direct continuation 

 with a surface ridge of the next lower internode, the same alter- 

 nating arrangement is apparent in the ridges of two successive inter- 

 nodes. The lateral branches are developed in the axils of the 

 scale leaves, but not having space to grow upwards they pierce the 

 narrow sheath. As a result of the reduction of the leaf laminse, 



