ANOMALOUS THICKENING IN DICOTYLEDONS AND GVMNOSPERMS. 615 



the form of a conical surface or of a disc, according to the inclination of the margins 

 of the crown ; in other words, the bundles form a middle layer in the stem, of similar 

 form to the whole crown, and similar in direction to the surfaces of it. Hooker calls 

 it the vascular stratum. It is, it is true, not directly proved, but hardly to be doubted, 

 that the individual bundles ai^ directly continuous into the leaves, we may therefore 

 call it the layer of the leaf-trace. Examined more accurately, it consists of two 

 layers of collateral bundles lying closely one above the other, and only separated by 

 narrow bands of parenchyma : the bundles turn their phloem portions, which are 

 strengthened by strong fibrous strands, towards one another, and their xylem portions 

 respectively towards the crown and the outer surface of the stem. Connections 

 between the two layers of bundles are probable, but not observed with certainty. 

 The bundles of the individual layer are placed with considerable regularity side by 

 side ; they have an undulating course, and are, as far as can be made out, here and 

 there laterally connected. 



In the central portion of the stratum of the leaf-trace, between the middle of the 

 crown and the insertion of the tap root, the course of the bundles is less regular, 

 according to Hooker's description ; here they form a tangled plexus, from which 

 descend the bundles of the root, to be more accurately described below. 



From the two layers of the stratum of the leaf-trace numerous much smaller bundles 

 branch at all points : these run in an oblique direction, on the one hand towards the 

 •whole surface of the crown, on the other towards the outside of the stem, being curved 

 in a sinuous manner apparently irregularly in all directions, and cpnnected by branches 

 one with another, thus forming a complex tangled net, which may be distinguished as 

 the peripheral network of bundles. The bundles of the net which run towards the 

 surface of the crown are often directly connected with those of the inflorescences. 



The bundles, which descend into the tap root, are sometimes connected with 

 those of the central portion of the stratum of the leaf-trace, sometimes with those of 

 the peripheral net, as branches derived from them : the main direction of their course 

 is, like that of the root, vertically downwards. In the transverse section of the root 

 they are arranged with some regularity in concentric rings, separated from one 

 another by zones without any bundles ; these resemble the rings in stems of Meni- 

 spermeae and Gnetacese. The rings are moreover the more regular, and the bundles 

 are larger the nearer they are to the middle. Hooker represents 5-7 such rings in 

 transverse sections of thin roots ; in a good young specimen before me, there are 

 eight rings at a point close below the stem, and where the radius of transverse section 

 was 30™™ : of these the outermost are very irregular, and the bundles small. The 

 bundles of each ring have an undulating course, and often anastomose laterally ; 

 there are also frequent oblique connections between successive rings, especially the 

 outer ones. The inmost ring surrounds, as a rule, a round pith-like central portion, 

 apparently without any bundles, which is for instance in the specimen before me 

 about 15mm broad. When carefully examined, however, it is not entirely free from 

 bundles, but contains a number of small strands of vessels, sieve-tubes, and fibres, 

 the arrangement of which cannot be more exactly described because of the nature of 

 the material. In some roots Hooker found in place of one inmost ring two eccentric 

 rings side by side, round which the outer ones were arranged with considerable 

 regularity, so as to form a simple system of rings. 



