448 MR. TV. C. WORSDELL ON THE COMPAEATIVE 



plant. The individuality of the leaf-trace bundle is destroyed soon after it enters the 

 xylem of the innermost zone, for it here gives off connections during its inward course to 

 the strands of this tissue, so that a complete transversely-sectioned bundle is not usually 

 observed in the larger meshes of the xylem-network as is the case in the phloem. 



The Fith. 



The individual bundles forming the complex medullary vascular system are similar in 

 form to the segments composing the vascular zones ffig. 4) ; those in the lower part of 

 the stem are, naturally, by far the largest and equal in size to those segments, consisting 

 almost entirely of secondary xylem and phloem, with a few small, irregularly-placed, 

 reticulate tracheides, -which represent the primary xyleui of these bundles. Several 

 radial rows of vascular tissue may be formed around a mucilage-canal separated by rows 

 of parenchyma-cells which represent medullary rays (fig. 6). 



Mohl * states that the medullary bundles of JSncepTialartos pass through the 

 medullary rays and become connected in the cortex with the leaf-trace bundles. This 

 statement, however, as I have been careful to determine, is an erroneous one. The 

 medullary bundles constitute a vascular system quite independent of that of the leaf- 

 traces. They penetrate the inner vascular zone by the medullary rays (fig. 7), their 

 tracheides uniting with the similar elements of that zone (fig. 8). This latter diagram- 

 matic figure illustrates remarkably well the point above referred to, for a leaf-trace 

 bundle from the cortex is here shown fusing with the same vascular zone and in close 

 proximity to the point of fusion of the medullary bundle, but, it is to be noticed, quite 

 independently of the latter. Pig. 7 represents a tangential section of the xylem of the 

 inner vascular zone with a medullary bundle lying obliquely in the ray and about to 

 fuse with the adjoining strands ; a mucilage-canal is seen to be accompanying the 

 bundle. 



Structure of the Hoots. 



The original primary tap-root of the plant had completely died away, and had been 

 succeeded by a number of adventitious roots springing from the flattened and partially 

 decayed lower end of the stem. 



It appears that these plants, which grow in clefts of rocks in the hilly country of South 

 Africa, have, at a certain season of the year, to endure a dry season, during which their 

 tap-root entirely dies away, along with, presumably, the foliage. During this period the 

 stem, swollen as it is with its massive parenchymeitous tissues and protected externally 

 by the thick and coriaceous leaf-bases, probably acts as a water- reservoir by means of 

 the network of mucilage- canals penetrating every part of the organ, the mucilaginous 

 secretion serving to retain every particle of moisture previously absorbed by the root, 

 and thus to preserve the stem from desiccation. It seems to me that this is a very 

 probable explanation of the presence of such a vast system of mucilage-secreting canals 

 in the stems of Cycads. It is a significant fact in this connection that the roots are 



* Vermischte Schriften, 1845, p. 200 



