Groom. — On Dischidia rafflesiana ( Wall .). 241 
towards the exterior. But the two classes of roots differ in 
the finer details in the mode of formation of cork. The 
dorsiventral structure of the attaching-roots is further ac- 
centuated by the fact that cork is not formed within the 
immediate region of contact (Fig. 9) ; and this is also true 
when contact is only ensured by the development of long 
root-hairs, as in the case quoted of the root spreading over 
a Moss. The object of this arrangement is obvious : the 
formation of cork on the side in contact would check the 
absorption of nutrient solutions. In pitcher-roots cork ap- 
pears to be formed only in their older parts, near the mouth 
of the pitcher, i.e. in regions where the root is most likely to 
become dry and where there would probably be no liquid to 
absorb. Here it is developed round the whole circumference, 
and secondary patches of phellogen originate within the bands 
of sclerenchyma. 
The vascular cylinder displays no general differences in the 
two varieties of roots, except that in some attaching-roots 
dorsiventral structure again appears (Fig. 11). In such a case, 
on the ventral side of the central mass of xylem, a few wood- 
vessels appear which greatly exceed all the rest of the wood- 
elements in the size of their lumina. That is, there is a group 
of larger vessels on the side on which the absorption of water 
is taking place. In this case the large size of these vessels 
cannot be attributed to differences of pressure ; if any difference 
of pressure existed at the time when the vessels were forming 
it would decidedly be greater on the ventral side, as can be 
seen from the tangential flattening of the young cortical cells 
which takes place on that side alone. These large wood- 
vessels are probably the result of a demand for more ready 
conduction of water on the side where absorption of liquids is 
taking place. 
In conclusion, I desire to express my thanks to Professor 
Vines for affording me the hospitality of the Botanical 
Laboratory of the University of Oxford, thus greatly facilitating 
the work involved in the foregoing investigation. 
