CYCADALES 



103 



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 coi^tex, 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 cylinder 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, d^, lateral traces 

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

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

 Helen Angela (59). 



