CYCADALES 



103 



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current statement has been that two leaf traces leave the vascular 

 cylinder on the side away from the leaf they are to enter, swing around 

 to left and right in wide curves through the cortex, forming the charac- 

 teristic girdles, and so pass to the leaf. This needs amendment 

 as follows. As usual in cycads, a variable number of scales succeed 

 the cotyledons before true foliage leaves are developed, but the traces 

 of scales and leaves are the same. In Dioon four strands leave the 

 central cylinder for each scale or leaf, at indefinite but well-distributed 

 points. Two of them leave 

 approximately on the side of 

 the cvlinder toward the leaf, 

 and without branching pass 

 more or less directly through 

 the cortex to the petiole. The 

 two other strands leave the 

 central cylinder approximately 

 on the opposite side, describe 

 wide right and left curves 

 about it through the cortex, 

 and so enter the petiole, where 

 they begin to branch freely 

 (fig. 81). The leaf traces of 

 the cycads, therefore, include 

 not only "girdles," but also 

 direct traces. The vascular 

 strands of both leaves and cotyledons are endarch at their junction 

 with the cylinder, but gradually become mesarch, and in the upper 

 stretches of leaf or cotyledon the exarch condition is approached 

 more or less completely. This transition from endarch to mesarch 

 and perhaps exarch seems to be a common feature of the foliar 

 strands of cycads. 



The origin of the "girdle" is a very obscure problem. In connec- 

 tion with Ceratozamia it was stated that the girdling became evident 

 in connection with the diameter increase of the inclosed group of 

 leaves and stem. It would be a natural inference that the curve of 

 the girdle is produced by the growth of the group of organs within; 

 but in Dioon it is evident that the girdle is established in the procam- 



FiG. 79. — Ceratozamia mexicana: dia- 

 gram of vascular plate of seedling, showing 

 origin of leaf bundles; d, </", lateral traces 

 of tirst leaf; e, e^, middle traces of same; 

 rest of lettering as in tig. 76. — After Sister 

 Helen Angela (59). 



