EQUISETINEAE. 



be inserted in a spiral line running round the stem ; this is actually the case some- 

 times in plants of abnormal growth; but in ordinary cases, owing to a small 

 displacement at a very early period, the three segments, which form a cycle on the 

 stem, arrange themselves so as to form a transverse disk of the stem, their outer 

 surfaces combining into an annular zone. Thus by unequal growth of the segments 

 in the longitudinal direction each cycle in the spiral formed by the segments gives rise 



FIG. 214. A longitudinal section of the extremity of the stem of an underground bud of Eqnisetum Telmateja ; 

 5 1 apical cell : xy first indication of an annular cushion for the formation of leaves, bb an older cushion of the same kind, 

 bs, bs the apical cells of a leaf-cushion already considerably advanced, rr rudiment of the cortical tissue of the internodes, 

 qq cell-rows from which the tissue of the leaf and its vascular bundle are formed, zV the lower cell-layers of the segments, 

 which take no part in the formation of the leaves. B horizontal projection of the apical view of a stem of Equisetutn 

 Telmateja ; S apical cell, / Fthe successive segments, the older ones further divided (VI should be IV), C horizontal 

 projection of the apical view of E. arvense. D optical longitudinal section of the extremity of a very weakly stem. 

 E transverse section of the extremity of a stem after the appearance of the vertical and first tangential walls. The Roman 

 ciphers indicate the segments, the Arabian the walls formed in them in the order of their succession, the letters the 

 principal walls of the segments. C, D, and E after Cramer. A natural size. 



to a whorl, which strictly speaking is not a true whorl because it is due to subsequent 

 displacement 1 . Each whorl of segments now produces a leaf-sheath and the inter- 

 node beneath it. While the three segments are being adjusted to form a transverse 

 disk of the stem, they undergo the divisions above described, which convert each of 

 them into a cellular body containing from four to six layers of cells. As soon as a 

 transverse zone is formed, the leaves begin to develope by the growth of the outer 



1 See what was said above on the development of the leaves in germinating plants. 



