644 MR JOHN M'LEAN THOMPSON ON 



traces are free within the cortex, they appear, in transverse section, as represented 

 in fig. 51. They merely reproduce, in miniature, the characters of the largest 

 leaf-traces. 



A leaf-trace of the transitional type, and springing from the side of the stele, is 

 represented in transverse section in fig. 50. It shows the re-formation of the layer 

 of stelar phloem below the departing leaf-trace, and the liberation of the leaf-trace 

 phloem from the stelar phloem. I am not aware of any leaf-trace departure which 

 is exactly comparable with the one described in these paragraphs, and for the present 

 we may consider it an unique type. 



The origin of the roots presents no points of special interest. The roots are 

 restricted to the lower surface and lower portions of the sides of the stele. They 

 originate, as do the leaf-traces, by a protrusion of the outer xylem-cylinder (figs. 1 

 and 52). When they have entered the cortex they appear in transverse section 

 as a solid mass of xylem, with small peripheral and larger central tracheides. 

 Around this xylem core is a narrow zone of phloem, a well-developed pericycle, and 

 an endodermis (fig. 53). A typical root-structure is not established until the cortex 

 has been traversed. When free from the cortex, the root has typically a diarch stele 

 (fig. 54). The pericycle and phloem are well developed, and the endodermis is of 

 large and delicate cells. A two-layered zone of sclerenchyma gives the fibrous 

 character to the mature roots, and the outer part of the cortex consists of two or 

 three layers of large parenchymatous cells, with which are associated numerous, 

 long, and often persistent root-hairs. There is nothing remarkable in the roots 

 except the persistency of the long root-hairs and the cortex. This persistency was 

 so marked a feature that perfect sections could be obtained from old roots of the 

 specimen which I had at my disposal. 



The largest leaf-traces pass into the large pinnate leaves and the smallest traces 

 typically supply the filiform leaves. The traces of intermediate size divide roughly 

 into two groups. The first group includes traces of moderate size which spring from 

 the upper portions of the sides of the stele, and, as a rule, supply pinnate leaves. 

 The second group is composed of traces springing from the lower portions of the 

 sides. Almost invariably they supply filiform leaves. The leaf-zoning has there- 

 fore found its explanation in the dispositions of the large and small leaf-traces upon 

 the stele. 



Passing now to the anatomy of the leaves, the outstanding features of the typical 

 simply-pinnate leaf may be considered first. The petiole is circular or oval in section 

 (fig. 14). The epidermis and the peripheral zone of cortex are markedly sclerotic, 

 but the inner cortex is composed mainly of large-celled parenchyma with thin walls, 

 and mucilaginous protoplasts. The protoplasts of the cortical sclerenchyma are 

 also highly mucilaginous, and the general condition of the cortex is indicative of 

 extremely xerophytic conditions. The trace is roughly oval in section, and the 

 endodermis is slightly indented on the upper surface. There is practically no 



