EQUISETACE^. 



[ 300 ] 



EQUISETACE.E. 



In the solid rhizome the centre is occupied 

 by cellular tissue of tolerably strong texture ; 

 outside this, as seen in a cross section, 

 stands a circle of air-canals, each surrounded 

 by a ring of vascular bundles ; next comes 

 a complete circle of vascular bundles com- 

 posed almost wholly of annular ducts ; be- 

 tween this vascular ring and the outside 

 lies parenchyma like that in the centre, 

 traversed by another concentric circle of 

 air-canals ; and immediately beneath the 

 epidermal cells there exists a layer of com- 

 pact blackish-brown parenchymatous cells. 

 When the rhizome is coated with hairs, 

 these are formed by development of the 

 epidermal cells into slender tubular pro- 

 cesses. Tracing the solid rhizome up to- 

 wards the points where the erect stems 

 arise, the central cellular substance is gra- 

 dually lost, and the outer portions are mo- 

 dified in their arrangement. The distribu- 

 tion of the air-canals and the vascular bun- 

 dles 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 papilhe, and especially re- 

 markable for the quantity of silica deposited 

 in their walls. This epidermis is studded 

 with variously formed stomata usually 

 arranged in double lines ; and the forms of 

 the epidermal cells and stomata are per- 

 fectly preserved in the siliceous ash which 

 remains after burning off the organic sub- 

 stance from a portion of this EPIDERMIS, 

 offering a curious microscopic object. Be- 

 tween 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, comes a circle of 

 masses, usually crescentic, of cellular tissue 

 containing chlorophyll. Next come usually 

 two concentric rings of air-canals, those of 

 the inner circle being individually sur- 

 rounded 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 j the structure of the 

 bundles is variable, exhibiting annular, spi- 

 ral, 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 com- 

 posed 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 groups of spoj-anyes, the 

 stalks of each adhering to the central axis, 

 so that we only see the upper side of the 

 cap externally (figs. 202, 203). This has 

 an angular border ; and the adjacent spo- 

 ranges being very close, the outer ends of 

 these bodies cause a tessellated appearance 

 of the whole in the earlier stages of de- 

 velopment. As the sporanges ripen, they 

 separate more from each other ; and when 

 one is removed (fig. 204), it is seen to 

 possess a number of little pouch-like cases 

 under the overhanging outer portion and 

 round the stalk ; these pouches burst by a 

 perpendicular slit inwards, and discharge 

 the spores. 



Fig. 202. 



Fig. 203. 



Fig. 205. 



Equisetum arvense. 



Fi<r. 202. Fruit-spike. Magnified 3 diams. 



Fig. 203. A spike halved vertically. Magn. 3 diams. 



Fig. 204. A stalked group of sporanges removed from 



preceding. Magn. 25 diams. 

 Fig. 205. A spore with elaters uncoiling. Magn. 200 



diams. 



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 j for the outer 

 coat splits up into four thread-like pro- 

 cesses (elaters), thickest and rather clubbed 



