388 CYCAS. 



Each year, as a rule, the apex of the shoot produces first a series 

 of scale-leaves (corresponding to leaf-bases with undeveloped 

 blades), then a series of foliage -leaves (which were protected by the 

 scales inside the bud). 



In the case of a female plant, the carpels (megasporophylls) are 

 clearly developed in place of several spirals, forming an apparent- 

 whorl, of ordinary leaves, for in the centre of the ' ' rosette " of 

 carpels the growing vegetative tip of the shoot may be seen, covered 

 by scale leaves In this respect Cycas itself differs from, and is 

 more primitive than, the other living Cycads, in whicli the carpels 

 are borne in female cones very similar in form to the male cones. 



546. Structure of Leaf. The structure of the leaf of 

 Cycas (and of other Cycads) is of great interest. Examine 

 transverse sections of (1) petiole, (2) leaflet. In the 

 petiole the bundles are arranged in a ring which on 

 cross section resembles the Greek letter omega, 12. These 

 leaf-bundles, which are also seen in transverse section of 

 a leaflet, are mesarch : on the outer side of each bundle 

 (the lower side in the leaflet) there is phloem, then some 

 parenchyma containing a number of tracheids, then the 

 protoxylem forming the apex of a well-marked triangular 

 xylem strand. 



That is, the protoxylem is in the middle of the xyleni, 

 and the greater part of the xylem is centripetal 

 (developed towards the centre of the petiole or the upper 

 side of the leaflet), the centrifugal xyleni (corresponding 

 to the whole xylem in the stem or leaf of a Dicotyledon) 

 being relatively small in amount. 



The cells of the spongy mesophyll of the leaf is modi- 

 fied on either side of the bundle, forming transfusion 

 tissue which spreads out horizontally from the xylem 

 and phloem of the bundle. 



The stomata on the underside of the leaflet are 

 peculiar ; the guard-cells of each stoma are arched over 

 by a cone-like outgrowth of the epidermis and cuticle. 



547. Normal and Coralloid Roots of Cycas. The 



root of Cycas resembles that of Yew in general structure, 

 but sometimes rootlets grow up to the surface of the soil 

 and branch in a coral-like manner. These "coralloid" 

 roots usually show, at about the middle of the cortex, a 



