EQUISETACE^. 



[ 242 ] 



EQUISETACE.E. 



and immediately beneath the epidermal cells 

 there exists a layer of compact blackish- 

 brown parenchymatous cells. When the 

 rhizome is coated with hairs, these are formed 

 by development of the epidermal cells 

 into slender, tubular processes. Tracing 

 the solid rhizome up towards the points 

 where the erect stems arise, the central cel- 

 lular substance is gradually lost, and the 

 outer portions are modified in their arrange- 

 ment. The distribution of the air-canals 

 and the vascular bundles varies; in some 

 cases, the peculiarities are even regular 

 enough to afford specific characters. The 

 surface is clothed by an epidermis composed 

 of elongated cells often elevated into papillae, 

 and especially remarkable for the quantity 

 of silica deposited in their walls. This epi- 

 dermis is studded with variously formed sto- 

 mates ordinarily arranged in double lines; 

 and the forms of the epidermal cells and 

 stomates are perfectly preserved in the sili- 

 ceous ash which remains after burning off 

 the organic substance from a portion of this 

 EPIDERMIS, offering a curious microscopic 

 object. Between the epidermis and the 

 central cavity, in a cross-section, lie, first, a 

 layer of thick-walled elongated cells, within 

 which, in the angular-stemmed species, come 

 a circle of masses, usually crescentic, of cel- 

 lular tissue containing chlorophyll. Next 

 come usually two concentric rings of air- 

 canals, those of the inner circle being indi- 

 vidually surrounded by annular ducts, and, 

 moreover, in some species a circle of 6-10 

 vascular bundles separates the inner from 

 the outer circle of air-canals ; the structure 

 of the bundles is variable, exhibiting annular, 

 spiral, and reticulated ducts. The inner 

 circle of air-canals lies in the parenchyma 

 which bounds the central cavity. At each 

 joint this cavity is cut off by a diaphragm 

 composed of three layers, in the intermediate 

 of which, of brownish cellular tissue, lies an 

 anastomosing ring, where all the vascular 

 bundles coalesce and give off branches to the 

 sheath (and branches when present). 



The club-shaped fruit-spikes consist of a 

 central axis forming the last joint of the 

 stem, on which are attached numerous 

 mushroom-shaped sporanges, the stalk of 

 each adhering to the central axis, so that we 

 only see the upper side of the cap externally 

 (figs. 206, 207). This has an angular bor- 

 der, and the adjacent sporanges being very 

 close, the outer ends of these bodies cause a 

 tesselated appearance of the whole in the 

 earlier stages of development. As the spo- 



ranges ripen, they separate more from each 

 other ; and when one is removed (fig. 208) 



Fig. 206. 



Fig. 207. 



Fig. 208. 



Fig. 209. 



Equisetum arvense. 



Fig. 206. Fruit spike. Magnified 3 diams. 



Fig 207. A spike halved vertically. Magn. 3 diams. 



Fig. 208. A sporange removed from the preceding. 



Magn. 25 diams. 

 Fig. 209. A spore with elaters uncoiling. Magn. 200 



diams. 



it is seen to possess a number of little pouch - 

 like cases under the overhanging outer por- 

 tion and round the stalk; these pouches 

 burst by a perpendicular slit inwards, and 

 discharge the spores. 



The spores of the Equiseta are very re- 

 markable, and unlike any other known ve- 

 getable structure. They are roundish cells, 

 with apparently only one coat, for the outer 

 coat splits up into four thread-like processes 

 (elaters), thickest and rather clubbed at their 

 free ends. While the spore remains on the 

 sporange, these fibres are rolled round the 

 spore ; but when the spores are discharged, 

 the coiled fibres uncurl (fig. 209), and assist in 

 scattering the spores, their elasticity causing 

 them to spring about. 



The Equiseta possess only this one kind 

 of spore, and the germination is analogous 

 to that of the Ferns, in which likewise only 

 one kind of spore exists. The membrane of 

 the spore pushes out a pouch-like process, 

 which after a time becomes cut off by a sep- 



