

n8 



BO TA XICA L GA ZE T TE 



[august 



the product of cambial activity, but in their vertical enlargement 

 and elongation it seems probable that these lignified elements 

 have simply been stretched out into their curious bending shapes 

 by the growth of the adjoining living parenchyma tissue. The 

 close scalariform markings on these vessels, in some cases approach- 

 ing almost a pitted character, would indicate that this growth or 

 elongation has taken place gradually, keeping pace with the longi- 

 tudinal expansion of the gap. 



It is also evident that the basal portions of many of these 

 vessels have their origin in quite a different manner. The charac- 

 ter of the pitting indicates that there has been a gradual ligniiication 

 of the ordinary parenchyma cells (fig. 4 B), and a subsequent 

 fusion by the breaking down and reabsorption of the partition 

 walls. The formation of the peculiar curved and bulbous-like bases 

 of many of these tracheids, where they come in contact with the per- 

 pendicular elements of the secondary wood, is shown in this way. 



Course of leaf traces in cortex 



The course of the fibrovascular bundles in the cortex, compli- 

 cated by the well known habit of girdling, was first described by 

 Karsten (7) in Zamia muricata, in 1856, later, in 1861, by Mette- 

 xius (10), and in more recent articles quite fully by Thiessex and 

 Sister Helen Angela in seedlings of Ceratozamia (3), Dioon edule 

 (11), and D. spinulosum (4). • 



A brief statement of the girdling situation in the embryo and 

 seedling of Dioon edule, as described by Thiessen, is approximately 

 as follows. For each leaf or scale leaf there are four distinct strands 

 leaving the vascular cylinder. Two of these leave on the same 

 side as the leaf for which they are destined, and pursue a direct 

 course through the cortex to the central part of the petiole without 

 branching; while the other two strands leave the cylinder approxi- 

 mately on the opposite side, describe a wide curve around it, and 

 finally enter the dorsal part of the leaf petiole, where they branch 

 repeatedly. , 



Helen Angela (4) has described a similar 





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pinii 





losum. 



agree 





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