398 



VASCULAR CRYPTOGAMS. 



been mentioned, disposed originally in a spiral with ^ arrangement ; and since 

 each segment without exception (as in Mosses) produces a leaf or what corresponds 

 to a part of a leaf-sheath, the leaves of Equisetum must also be arranged in a 

 spiral. This does, in fact, sometimes occur when the growth is abnormal ; but 

 when the growth is normal, a small displacement takes place at a very early 

 period, of such a nature that the three segments which form a cycle always 

 come to be arranged into a disc transverse to the stem, their outer surfaces thus 



Fig. 279.-/4 longitudinal section of the end of a stem in an underground bud oi Eqtcisettini Tehnateia ; S apical cell, xy first 

 indication of the girdle from which the leaves are subsequently formed, b b <\. more advanced and distinctly marked foliar 

 girdle, bs the apical cells of a strongly projecting foliar girdle, rr rudiment of the cortical tissue of the internodes, ^.^ rows 

 of cells from which the leaf-tissue and its fibro-vascular bundle proceed, ?« the lower layers of cells of the segment which take 

 no part in the formation of leaves (from nature) ; B horizontal projection of the apical view of the end of a stem oiE. Tehnateia; 

 s apical cell, I — /^the successive segments, the older ones still further divided ; C horizontal projection of the apical view of 

 K. arvense; D optical longitudinal section of the end of a very slender stem ; E transverse section of the end of a stem after the 

 formation of the vertical and first tangential walls. (C, A -£'. after Cramer ; the Roman numerals indicate the segments, the 

 Arabic numerals the walls formed in them in the order of their succession ; the letters the primary walls of the segments.) 



forming an annular zone or girdle. According to Reess, to whom this observation 

 is due, the three segments of each cycle are formed in rapid succession, while 

 a longer time elapses between the formation of the last segment of the preceding 

 and that of the first of the succeeding cycle. Thus by the unequal growth of 

 the segments in longitudinal direction each cycle of segments or turn of the 

 spiral produces a whorl, which therefore, strictly speaking, is a pseudo-whorl, 

 because resulting from subsequent displacement. Each whorl of segments now 



