THIRD GROUP. — VASCULAR CRYPTOGAMS. 



The growing end of the stem sometimes projects some way beyond the point of 

 origin of the youngest leaves, and then appears naked, as in Polypodium vulgare, 

 P. sporodocarpum and other creeping species ; the same may be observed in Plen's 

 aquilina, where in old plants according to Hofmeister the growing end is often 

 without leaves for the space of several inches ; in many Hymenophyllaceae Mettenius 

 informs us that such leafless prolongations of the stem have been taken for roots. 

 In other cases on the contrary, especially in species that grow erect, the longitudinal 

 growth of the stem is much slower and its apex is concealed in a leaf-bud. The apex 

 of the stem is often flat, and sometimes, as in Pteris, it is even sunk in a funnel- 

 shaped projection of the older tissue (Fig. i6o E). The apex of the stem is always 

 occupied by a distinct apical cell, which either divides by walls alternately inclined, 

 and then resembles when seen from above the transverse section of a biconvex lens, 

 or it is a three-sided pyramid with a convex anterior surface and three oblique lateral 

 surfaces which intersect behind. The bounding lines of the segments, which in the 

 first case are formed in two rows, in the second in three rows or with more 

 complicated divergences, soon disappear in consequence of the numerous cell-divisions 

 and the distortions caused by the growth of the masses of tissue around the apex and 

 that of the leaf-stalks. The apical cell, for example, in Pkris aquilina has the shape 



of a wedge with two sides, and its segments 

 on the horizontal stem form a right and a 

 left series ; the edges of the cell are turned 

 upwards and downwards (Fig. 157); the 

 same is the case, according to Hofmeister, 

 in Niphobolus chiiiensis and N. rupestris, in 

 PoljypodiuTH aureum ■SiwdiP . pimctatiim, and in 

 Plaiycerium alcicorne; in Polypodium vul- 

 gare, according to the same authority, the 

 apical cell is sometimes two-sided, some- 

 times a three-sided pyramid ; the last is 

 its form in Aspidium Filix-mas and some 

 others. We may for the present take it 

 to be the rule, that creeping stems with 

 bilateral structure have a two-sided apical 

 cell, and that in erect or ascending stems with rosettes of leaves radiating in every 

 direction the apical cell is a three-sided pyramid \ 



The details of the genetic relations of the segments of the apical cell of the 

 stem to the formation of the leaves and of the stem itself are not yet clearly 

 ascertained. There is no doubt that each leaf originates in a single segment, and 

 that this segment-cell is applied to the formation of a leaf very soon after it is itself 

 formed ; Kny states that a leaf proceeds from every segment in Ceratopteris, which 

 agrees therefore in this respect with the Mosses ; but this is certainly not the case 

 with most Ferns. 



The phyllotaxis sometimes corresponds to the formation in straight rows of the 

 segments of the stem ; thus the two rows of leaves in Pteris aquilitia, Niphobolus 



Fig. 157. Apical view of tlie extremity of the item 

 Pteris agiiilina ; y the apical cell of the stem, x apical cell 

 the youngest leaf, hh hairs covering the apical region which 

 surrounded by a cushion of tissue. 



' [Klein, Vergl. Unters. ii. Organb. u. Wachsth. am Vegetationsp. dorsiventral. Fame Bot. Ztg. 

 0.] 



