EQUISETACE^. 77 



their base. The habit of the plant depends on this mode of 

 formation of the leaves, and on the verticillate arrangement of the 

 branches, which spring from buds in the cortex. The stem, 

 rhizome, and root are all derived from a single apical cell, which 

 divides into three series of segments. In the internal structure of 

 the stem the air-cavities are of great importance. Its centre is 

 occupied by a large central air-cavity (PI. IX., Fig. i, /'), and in 

 the surrounding ring of tissue, which is often rather narrow, there 

 is almost always a cortical air-cavity (Fig. i, /) between each pair 

 of vascular bundles. In addition, there often occur also the 

 so-called essential air-cavities (Fig. r, /") in the vascular bundles. 

 The stomata (Fig. i, sf.) are usually placed in a single or in 

 several rows between the elevated ridges of the stem. The 

 cortex consists of thin-walled or of only moderately thick-walled 

 parenchyma. It is separated on the inside from the vascular 

 bundles by a sheath, which sometimes encloses all the bundles 

 together (Fig. i, s.). The vascular bundles ascend in a vertical 

 direction and parallel to one another through the internodes, and 

 form annular coils in the nodes ; they are always "closed" bundles, 

 containing no cambium. Two other groups of vascular cells are 

 found on the cortical side of the bundle. The bast-portion 

 contains three elements : — parenchyma, bast-fibres, and sieve- 

 tubes ; it lies between the four groups of vascular cells and the 

 vascular bundle-sheath. 



In the root is an axial bundle of vascular cells surrounded 

 by elongated parenchymatous cells, with which sieve-tubes and 

 bast-fibres are intermixed. The sporangia are capsules placed on 

 the under-side of scales belonging to the fructification (Fig. 2, /). 

 The fructification or receptacle is often placed on special shoots, 

 which are distinguished by their external form and by their brown 

 colour from the sterile green stems. The spores (S.S.) are pro- 

 vided with two hygroscopic bands or elators (Fig. 2, S.e.)^ only 

 loosely attached to them, formed by the splitting into narrow 

 strips and the partial detaching of the exospore or outermost of 

 the three coats of the spore, and serving, by their hygroscopic 

 properties, to assist in their dissemination. On germination, the 

 spore gives rise, first, to a flat prothallium, upon which are pro- 

 duced the antheridia and archegonia (PI. X., Figs, i, 2, 3). Most 



