ORIGIN OF RADIAL AND COLLATERAL STRANDS 861 



the special photosynthetic apparatus is most highly differentiated. 

 The palisade-tissue may, in fact, be said figuratively speaking 

 to exercise an attraction upon the hadrome-portions of the vascular 

 bundle, and thus to cause that disturbance of the concentric arrange- 

 ment which produces first the excentric and finally the collateral type 

 of structure. This " attraction " is undoubtedly closely connected with 

 the fact, that it is the hadrome which supplies the photosynthetic tissue 

 with water and mineral salts. The outward flow of these substances 

 from the vascular bundles is mainly directed towards the upper surface 

 of the leaf, and would thus be impeded if leptome were developed on the 

 adaxial side of the bundles. In order, therefore, to ensure direct com- 

 munication between hadrome and palisade-tissue, the former shifts 

 upwards, and in the end entirely displaces the leptome which originally 

 occupied the adaxial margin of the concentric bundles. The outward 

 diffusion of water and nutrient salts, of course, takes place chiefly in the 

 smaller bundles, a circumstance which explains why only the lesser 

 veins become collateral in Fern-leaves. . 



According to the view set forth above, the radial and collateral j 

 types of bundle have both been derived from the primitive concentric! 

 condition under the influence of the same principle. In either case, the 

 change of structure facilitates the diffusion of water and mineral salts, 

 which in the root have to pass from the cortex to the hadrome, and in 

 the leaf are transferred from the hadrome- to the palisade-tissue. 

 The radial symmetry of the root entails a centrifugal displacement of 

 the hadrome along several radii, whereas in the dorsiventral leaf the 

 water-conducting tissue always shifts in the adaxial direction ; thus ( 

 totally different derivative types of vascular structure result in the two 

 cases. 



The occurrence of collateral vascular bundles with adaxial hadrome 

 in sepals, petals, bracts, bud-scales, etc., as well as in dorsiventral foliage- 

 leaves, is probably to be regarded as the result of an inherited tendency, 

 depending on the fact that these various structures are in all likelihood 

 phylogenetically derived from foliage-leaves. 



It has already been stated that the stems of Dicotyledons and Gym- 

 nosperms and also those of Equisetum contain a circle of collateral 

 bundles with external leptome ; here phylogenetic considerations throw 

 some light upon the probable origin of the collateral condition. In all 

 the above-mentioned cases, namely, the whole circle of vascular bundles, 

 the pith enclosed thereby, the primary medullary rays which divide the 

 bundles from one another and the pericycle which separates them from 

 the endodermis (or starch-sheath) collectively represent a single central 

 cylinder homologous with the central cylinder of a root or a young 

 Fern-stem. The problem thus resolves itself into an enquiry concerning 



