﻿FOLIAGE. 



93 



Turning briefly to the existing cycads it is, however, instructive to note one of 

 the least modified forms of pinnule structure, as seen in Bowenia spertabilis, with 

 bipinnate fronds and an exceedingly thin pinnule blade. (See fig. 50.) Here both 



Fig. 50. — Bowenia speclabilis. ■ 125. Trans- 

 verse section o( pinnule, showing a simple 

 bundle form with centripetal xylem (x 1 ). 

 centrifugal xylem (x"), and a more or less 

 distinctly double bundle sheath (b). 



the dorsal and ventral epidermis 

 are alike formed by a character- 

 istic single layer of cells with 

 heavy outer and somewhat thin- 

 ner inner walls. The bundle 

 sheath consists of a double layer 

 of cells, the outer layer being 

 larger and thicker than the 

 inner, just as in Cycadeoidea 

 i/igens, with occasional very 

 heavy-walled cells scattered 

 about the outer sheath or in the 

 surrounding parenchyma. The 

 minute bundle is of mesarch col- 

 lateral structure, with the cen- 

 tripetal xylem cells of larger 

 size than, but nearly the same 

 number as, the cells of the 

 well-marked centrifugal xylem, 

 neither xylem nor phloem being 

 radially arranged. Beyond this 

 simplest type there is, however, 



Fig. 51. — Cycadeoidea ingens. T. 208. Transverse section of partly 



emergent but still folded frond deeply embedded in ramentum. )- about 4. 



Arrow points toward axis of trunk. The rachis lies to the nether side. 



This section was polished without removal from the trunk. The siliceous matrix of 

 the ramental hairs and pinnules is shown in solid black : the dermal layer as 

 a narrow white band ; the heavy upper layer of palisaded hypodermal scler- 

 enchyma by a heavy, and the thin lower sclerenchyma by a light continuous line ; 

 the vascular bundles by a light line marking the bundle sheath and its connection 

 with the hypodermal sclerenchyma. A few cells appear to project from the 

 lower hypodermal sclerenchyma between the bundles, as in various recent cycads, 

 but the series of lines indicating such should not be shown as if crossing the entire 

 thickness of the parenchyma. The section cuts the folded frond iust above the 

 tip of the rachis, above which there hence project 34 pairs of pinnules. The 

 total number of pairs may have beenlessthan 50. (Cf. photograph 8, plate XIX.) 



in the various existing cycads a long series of modifications in the presence, amount 

 and grouping of hypodermal bands of sclerenchyma, the presence or absence of pali- 

 sade parenchyma, the general character of the ground parenchyma and of the bundle 

 sheaths, and in the disposition and amount of centripetal and centrifugal xylem and 

 of the phloem. The most leathery and lignified forms are to be seen in Encephalartos, 

 certain forms of Zamia, etc. ; and if one combines the main characters of Encepha- 

 lartos cycadifolius, which has palisade parenchyma above and a heavy sclerenchyma 

 zone below the bundle sheaths, with those of Zamia furfur acea, which has a bundle 

 sheath confluent on its xylem side with the hypodermal sclerenchyma, a replica of the 



