534 STUDIES IN FOSSIL BOTANY 



be compared to the accessory transfusion-tissue which 

 occurs in the genus Cycas} In this species of Cordaites, 

 the mesophyll shows little differentiation. The most 

 interesting point is the structure of the bundles them- 

 selves, which agree exactly with those in the leaves of 

 recent Cycads. The xylem is in two parts, with the 

 spiral elements (J) between them. The larger portion 

 is towards the upper surface (centripetal, a), and the 

 smaller towards the lower surface (centrifugal, b). 

 Below this again is the phloem, usually ill preserved. 

 In a word, we have in these cases (Fig. 192, A and C) 

 the collateral, mesarch structure, characteristic of the 

 bundles of Cycadean leaves, and already familiar to us 

 in other fossil plants. Some other species show the 

 same structure of the bundles, but in several the 

 centrifugal part of the xylem is absent, as in the 

 petioles of Medullosa, whereas it only dies out in the 

 finer bundles in the case of recent Cycadean leaves. 2 



In other anatomical points there is considerable 

 diversity of detail. The mesophyll is in some forms 

 clearly differentiated into an upper palisade layer, and 

 a lower lacunar portion (see Fig. 192, B and C). This 

 is beautifully shown in a species investigated by Miss 

 Stopes, and identified as C. principalis. In the same 

 species Miss Stopes observed the very interesting fact 

 that both the inner and outer sheaths surrounding each 

 bundle consist of elements with bordered pits, and may 



1 A reticulate system of thick-walled tracheides, which extends from 

 the midrib to the margin of the leaflet in this genus. See Worsdell, " On 

 Transfusion-Tissue," Trans. Linn. Soc. 2nd ser. (Bot. ), vol. v. 1897, 

 p. 308. 



2 See Miss M. C. Stopes, " On the Leaf- Structure of Cordaites,'' New 

 Phytologist, vol. ii. 1903, p. 91. 



