ABSORPTION-CELLS ON LEAVES. 



233 



forms a dry crust over the glands and protects their tender-walled cells from too 

 great evaporation. 



Very remarkable also are the structures adapted to absorption on the leaves of 

 saxifrages belonging to the group Aizoon, and on those of a large proportion of the 

 Plumbagineae. The saxifrages in question have little depressions visible to the 

 naked eye upon the upper surface of the leaves behind the apex, and along the 

 margins. When the margin is dentate or crenate, as, for instance, in Saxifraga 



Fig. 55.— Absorptive Cavities and Cups on B'oliage-leaves. 



Leaf from a slioot of the Aspen, ^jlie base of this leaf; x3. s Section through an absorption-cup; x25. < Leaf of 

 Acantholimon Senganense. 6 Section through part of this leaf; xllO. « Leaf of the Evergreen Saxifrage {Saxi/raga 

 Aizoon). 1 Two teeth from the margin of this leaf. The absorptive cavity in the upper tooth incrusted with lime; the 

 lower one with the incrustation removed. ' Section through a tooth from the leaf and its absorptive cavity ; x 110. 



Aizoon (see fig. 55^), one of these cavities occurs in the middle of each tooth. 

 The cells forming the outer edge of the tooth or scallop are always much 

 thickened, firm, and rigid; but the median portion of the leaf as a whole is fleshy, 

 and composed of a bulky large-celled parenchyma. The vascular bundle, after 

 entering the leaf at its base, divides into a number of lateral bundles which eitlier 

 run towards the margin without further ramification (as in Saxicwsia), or else 

 form a net-work by uniting one with another in their course (as in Saxifraga 

 Aizoon). These lateral bundles terminate in the marginal teeth of the leaf and 

 immediately beneath the little cavities which occur there, whilst the extremit}'- of 

 each bundle swells into a knob or pear-shaped enlargement strongly resembling 

 the roundish groups of spirally-thickened cells in the tentacles of the Sun-dew 



