68 MINUTE MARVELS OF NATURE 



circumstances in life ; and in Fio'. 45 is shown a 

 section of the fleshy leaf of one of the stonecrops 

 — plants which grow in dry, sandy, or stony 

 situations, and develop thick, fleshy leaves, like 

 short stalks in clusters, so as to retain moisture 

 and prevent evaporation when exposed to the 

 heat of the sun's rays. Many desert plants like 

 cactuses, euphorbias, acacias, &c., have dispensed 

 altogether with true leaves, their functions being 

 fulfilled by the thick fleshy stems ; though it is 

 sometimes perplexing to decide where leaves end 

 and stems begin. It will be seen in the stonecrop 

 that the epidermal structure is thickened and 

 strong, and that the internal tissue is more or 

 less uniform, in comparison with the previous 

 leaf-structures. 



As another example of a different form of leaf, 

 a section of the curious awl-shaped leaf of the pine 

 is represented in Fig. 46. The epidermis is also, 

 in this case, thick-walled, because the pine, being 

 an evergreen like the laurel, requires protection 

 in winter. The mid-rib in this leaf consists of 

 the two vascular bundles or central veins, which 

 show distinctly in the illustration, and which are 

 but the continuation of the leaf stalk. The 

 straight palisade cells are in this instance re- 

 placed by others of sinuous outline, to contain 

 the green chlorophyll grains ; while the tubes 



