EQVISETTNEJE. 



401 



to the light. It may be assumed that there is always as large a number of buds 

 in a rudimentary condition as there are sheath-teeth. On the erect leafy stems 

 of B. Telmateia, E. arvense, and other species, they all attain complete development, 

 and produce the numerous slender green leafy shoots of these species; in other 

 species the development of the branches is more sparing; some, as E. hiemale, 

 usually form no aerial lateral shoots at all except when the terminal bud of the 

 stem is injured, and then the node next below produces a shoot. Branches do 

 not usually make their appearance on rhizomes in the form of complete whorls, 

 but in twos or threes, but on the other 

 hand they are more vigorous ; they be- 

 come either new rhizomes or ascending 

 stems. Since, in the cases first mentioned, 

 the buds arise like the leaves in strict 

 acropetal succession, it may be assumed 

 that where the production of shoots is only 

 induced at a later period by accidental 

 circumstances, the buds have up to that 

 time remained dormant in the interior. 



The Roofs arise in whorls, each im- 

 mediately below a bud ; but they may also 

 often be suppressed, and may be deve- 

 loped, according to Duval- Jouve, even on 

 aerial nodes, by humidity and darkness. 

 Their development has been studied by 

 Nageli and Leitgeb (/. c.) ; in its earliest 

 stages, which are represented diagram- 

 matically in Fig. 284, it resembles essen- 

 tially that of Ferns. The cortex is differ- 

 entiated into an inner and an outer layer ; 

 the former forms air-conducting intercel- 

 lular spaces, at first arranged, like the cells 

 themselves, in radial and concentric rows, 

 and afterwards combining by the rupture 

 of the cells into a large air-cavity sur- 

 rounding the central fibro-vascular cylinder. 

 As the fibro-vascular cylinder of the root 

 developes (seen in transverse section), 

 each of the three primary cells which 



alone of the six reach the centre is first of all divided by a tangential wall, so that 

 the rudiment of the vascular bundle now consists of three inner and six outer cells. 

 The six outer cells produce a cambial tissue in which the formation of vessels 

 begins, commencing from two or three points of the circumference and advancing 

 towards the interior. Last of all one of the three inner cells forms a broad central 

 vessel; and phloem is produced in the circumference of the vascular cylinder. In 

 the other vascular Cryptogams the innermost layer of the cortical tissue forms 

 the bundle- sheath {Pleromscheide, Schutzscheide), the radial walls of the cells 



Dd 



Fig. 283. — Longitudinal section tlirougli an underground 

 bud oi Eqiiisetuni arvense; ss apical cell of the stem, li — gb the 

 leaves; K K' two buds; the horizontal lines across the stem 

 indicate the position of the diaphragms. 



