ASSIMILATING STEMS 167 



water-storing tissue is encountered in stem -succulents (e.g. Cactus) . 

 Through the loss of water, by transpiration and absorption by 

 the adjacent assimilatory cells during periods of drought, the 

 cells of the aqueous tissue shrink, and this results in the walls 

 becoming thrown into small folds which disappear, during the 

 wet season, as the plant regains turgidity. Loss of water from 

 such aqueous tissue is often retarded by the presence of thin 

 mucilage in the cell-contents which exudes from a broken surface 

 as a slimy fluid (e.g. in the Ice-plant, Mesembryanthemum). 



Attention has already been drawn to the presence of chloro- 

 plasts in the outer cortical cells of young stems (cf. p. 88) ; 

 but, where the leaf-surface is small, the stem may retain its 

 assimilatory powers for some years after the inception of 

 secondary thickening, and may even become enlarged by wing- 

 like outgrowths whereby its efficiency is increased. The Broom 

 (Cytisus scoparim), the Whortleberry (V actinium myrtillus, 

 Fig. 87), and the Gorse (Ulex) furnish examples of such assimi- 

 latory stems. 1 In the first-named downward prolongations from 

 the margins of the leaf -bases give the stem a ridged appearance. 

 A cross-section shows the usual thick-walled epidermis beneath 

 which, in each ridge, there is a strand of fibres; but, except for 

 these, the whole periphery of the cortex consists of a layer of 

 palisade tissue succeeded by four or five layers of closely packed 

 cells, also containing chloroplasts. The stomata occur at in- 

 tervals throughout the parts of the epidermis overlying the 

 assimilatory tissue. In other respects the stem exhibits the 

 normal structure of a secondarily thickened axis. 



The leaves of the Whortleberry, though not greatly reduced, 

 are usually deciduous, so that during the winter the plant only 

 assimilates by means of its winged stem. Beneath the epidermis 

 is a continuous zone of assimilatory tissue (Fig. 87, as.), con- 

 sisting of rounded or polygonal cells, uninterrupted by fibres. 

 The inner cortex is formed by a network of chlorophyll-containing 

 cells in which the meshes are occupied by large aqueous ele- 

 ments (aq.). 



It has been possible, in the preceding pages, to recognise a 

 more or less intimate relation between the construction of the 

 plant and the circumstances under which it lives. The effect 

 1 See also F. & S., p. 175. 



