EQUISETACEM. 



371 



node next below produces a shoot. Branches do not usually make their appear- 

 ance on rhizomes in the form of complete whorls, but in twos or threes; but on 

 the other hand they are more vigorous and become either new rhizomes or ascendino- 

 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 Rools arise in whorls, each immediately below a bud ; but they may also 

 often be suppressed, and may be developed, 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 dia- 

 grammatically in Fig. 278, it resembles essentially that of Ferns. The cortex is 

 differentiated into an inner and an outer layer; the former forms air-conducting 



^ .? s / r c 



Fig. 278.— Diagram of the succession of cell-divisions in the apex of the root of F.qtiisetnm hyevtale (after Nageli and Leitgeb) 

 (this diajfrani will serve also in the main for Ferns and for Marsilea). A longitudinal section ; B transverse section at the lower 

 end of ^; hhh the primary walls, sss the walls of the sextant-segments, indicated in ^ by the figs. /—-l'/-'/, kl7nnpW\& layers 

 of the root-cap, all the further divisions being omitted \ c c \x\ the interior of the root indicates the cambium-walls by which the 

 primary fibro-vascular bundle is divided from the cortex of the root, e the boundary-wall between the epidermis o and the cortex 

 (epidermal wall), r r boundary-wall between the outer and inner cortex (cortical wall), i, 2, 3, the successive tangential walls by 

 which the inner cortex is divided into several layers, the radial divisions being omitted. 



intercellular 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 

 surrounding the central fibro-vascular bundle. As the fibro-vascular bundle 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 bundle. The 

 branching of the root is, as in Ferns, strictly monopodial or acropetal ; but since 

 there is here no ' pericambium,' the lateral roots arise in contact with the outer 

 vessels. 



B b 2 



