100 TRANSACTIONS OF THE [Sess. lxxx 



laineux fait reelleinent defaut." This states a critical 

 difference recognisable at sight betwixt RJl. lactewnn, 

 Franch. and Rh. lacteum, var. macrophylluin, Franch. 



The indumentum of Rh. lacteum, Franch. forms a uniform 

 smooth velvety dull fawn-coloured covering to the leaf 

 under surface and when looked at closely shows prismatic 

 scintillate points all over. It is composed of tufts of hair- 

 cells. Each tuft has a very short base of attachment the 

 cells of which have a yellow-brown content. From the 

 base spread out thin-walled unicellular branches, some four 

 or five, of no great length. The}' are wide and empty, 

 somewhat vesicular, and colourless. These tufts are close 

 set and their branches closely interlock. The walls of 

 these cells give the prismatic reflections on the surface ol 

 the indumentum. Many tufts form one stratum of 

 indumentum. 



In Rh. lactewni, var. macro phijllum, Franch. the indu- 

 mentum of the under surface of the leaves produces a 

 hazel- brown covering which under moderate magnification 

 — even to the unaided eye — appears to be coarsely pitted. 

 It is not smooth and velvety but somewhat fluffy and does 

 not show prismatic scintillations. It is composed of cup- 

 shaped scales each with a definite many-celled stalk 

 expanding into a membranous cup one cell thick showing 

 a network of the walls of the component cells. The rim 

 of the cup is undulate and runs out at points into long 

 tortuous threads which are intricatel}' woven between the 

 mouth of the cups. The tint of the cells of the cups gives 

 the colour of the indumentum. But these cup-shaped 

 cells at the surface of the indumentum are not the only 

 ones. Beneath these and of all sizes down to (juite few- 

 celled almost unformed ones are other colourless scale hairs 

 which, when as often happens tiu; brown scales of the 

 free indumentum surface; fall off", appear as a greyish lower 

 stratum of indumentum taking the place of the scales 

 removed. Tiie indumentum here is then of more than one 

 stratum. 



In the ordinary language; of systematists the covering- 

 would be called a tonu^ntum in both cases and tlie under 

 surface of the leaf ha described as tomentose. But in the 

 crcnus Rh.odod/'.ndfon tliere nvc. rnanv kinds of indumenta 



