CYCADALES 



III 



hypodermal cells. The epidermis is strongly cutinized, as in conifers, 

 and the stomata, which in all the genera except Bowcnia occur only 

 on the under surface, are deeply sunken. In most forms the mesojihyll 

 is differentiated into palisade and spongy parenchyma, both rich in 

 chlorophyll, but in addition to this some leaves, as in Cycas, have 

 colorless cells, elongated parallel with the leaf surface, lying between 

 the palisade and the spongy parenchyma. A general view of the 

 structure of a leaflet is shown in figs. 90, 91. 



Fig. 90. — Cycas revoluia: part of transverse section of pinnule, showing the 

 cholorophyll-bearing palisade layer, the elongated colorless cells in the middle region, 

 beneath this a few layers of mesophyll, and the lower epidermis with two stomata. 



Fig. 91. — Dioon edide: part of transverse section of pinnule, showing palisade 

 layer, lower mesophyll region without elongated colorless cells, and lower epidermis 

 with two stomata. 



The first leaves of the seedling are almost always scale leaves, one 

 or more of which form a protection for the first foliage leaf. Suc- 

 ceeding leaves are similarly protected, and when crowns begin to be 

 produced each young crown is covered by a large number of thick, 

 brownish, hairy scale leaves. Young cones are similarly protected, 

 the scale leaves covering the young staminate cone of Dioon edule 

 being as thick, fleshy, and hairy as the ovulate sporophylls. and in 

 such cases they form an appreciable part of the armor. In some 

 species of Macrozamia there are no scale leaves at all. 



