THE GYMNOSPERMAE : CYCADALES, ETC. 717 



pith, xylem ring, cambium and outer phloem ring. The xylem elements 

 differ from those of the stem bundles in being scalariform, as in Ferns. As 

 these girdle systems are permanent, they must expand in conformity with 

 the secondary growth of the stem, and their xylem elements become pro- 

 gressively longer as the stem grows. These girdles must play no small part 

 in strengthening the trunks of old plants. It is noteworthy that in the seedling 

 there is no girdle system and the leaf traces take a direct course from stele to 

 petiole. This is probably the more primitive arrangement. Girdles are also 

 absent from the cones. 



A leaf trace at its point of entry to the petiole is normally collateral, with 

 the xylem on its upper side, as in the Conifers. The xylem consists of regular 

 rows of tracheids separated by medullary rays and is partly primary, partly 

 secondary in origin, but is all centrifugal, with an endarch protoxylem. 

 Higher up the petiole, however, this xylem gradually diminishes and another, 

 centripetal, mass of xylem appears on the other side of the protoxylem (Figs. 

 720 and 721). Throughout the greater part of the frond the bundle contains 

 a large mass of this centripetal xylem with only a small remnant of the 

 centrifugal wood, which is separated from the protoxylem by parenchymatous 

 cells. Bundles with both types of xylem are well known among Vascular 

 Cryptogams, but are otherwise almost unknown in Spermatophytes. They 

 are called mesarch (Fig. 722). In the Cryptogams, however, both xylems are 

 primary, while in Cycas it is not certain that the centrifugal xylem in the 

 petiole bundles is truly primary. It may be at least in part a remainder of 

 the secondary xylem of the trace bundle. In Staugeria it does seem to be 

 truly primary, but in other Cycads the term mesarch may not properly be 

 applied as that signifies a homology with the cryptogamic type which is 

 unproven. The term diploxylic is preferable. 



Anatomy of the Leaf 



The lamina of the Cycas leaf is leathery and thickly cutinized. The 

 epidermis and the upper hypodermis are highly thickened, and the stomata, 

 on the lower surface, are sunk in pits with overarching rims. The mesophyll 

 has a well-developed palisade layer on its upper side and the lower part of 

 the mesophyll consists of parenchymatous cells with rather scanty intercellular 

 spaces, corresponding to the spongy mesophyll in Angiosperms (Fig. 72'^). 

 Between the palisade layer and the lower mesophyll there is a layer, three 

 or four cells thick, of long colourless cells which run transversely from the 

 midrib to near the margin. As the midrib is unbranched these cells probably 

 form a conducting channel for water and are called the transfusion tissue 

 (cf. Piniis, p. 673). The whole external aspect of the leaf is similar to that 

 of some Palms with which Cycads were originally classified on account of 

 this superficial resemblance. 



