232 



ABSORPTION-CELLS ON LEAVES. 



similar contrivance is also exhibited by the leaves of alpine roses and those of the 

 American Bacharis. For instance, on the under surface of the leaves of the Alpine 

 Rose (Rhododendron hirsutum) there is a large number of discoid glands (fig. 54 5 ), 

 each of which is supported on a short stalk and sunk in a little hollow (fig. 54 p ) 

 The cells composing the gland are arranged radially, and contain slimy, resinous 

 matters capable of swelling up. These contents are also excreted, and then cover 

 the entire glandular disc, and often even the whole surface of the leaf in the form 



Fig. 54. — Absorption of Water by Foliage-leaves. 



1 Grooved rachis of the ash-leaf. 2 Section through the same ; x 30. 3 Peltate group of cells from the groove. * Section 

 through the base of a leaf of the Dwarf Gentian ; x20. 6 Under side of a leaf of Rhododendron hirsutum ; x30. ^Section 

 through a leaf of Rhododendron hirsutum. 



of a light-brown crumbly crust. When drops of rain fall upon Alpine Rose leaves, 

 the whole of the upper surfaces, in each case, is in the first place moistened; but 

 without delay, and partly through the action of the hairs fringing the margin, the 

 water soaks on to the under side of the leaf. As soon as it reaches the glands it is 

 taken up by the crumbly incrustation mentioned above, which swells up in con- 

 sequence. The little cavities in which the glands are situated also fill with water, 

 and each gland is then immersed, as it were, in a bath, and able to absorb as much 

 moisture as is required. Owing to the glands being invariably developed above 

 the vascular bundles of the leaf (see fig. 54 6 ), the water that is absorbed can be 

 conducted without delay by them to the places where it is required. As soon as 

 the leaves of alpine roses become dry again, the mass of resinous mucilage again 



