i8 93 . RAINFALL AND THE FORMS OF LEA VES. gg 



roots from being dried up. The later leaves are erect and much 

 divided. We find the same contrivance in one of the aroids, Pothos 

 aurea, the stem of which is at first creeping, and bears flat sessile 

 undivided leaves to protect the roots ; when the stem ascends the 

 leaves become stalked and pinnate. There are numerous examples 

 of heterophylly among our European herbs ; in many of the 

 Compositae, Cruciferae, etc., the upright stalk bears linear, pinnate, or 

 lobed leaves, while those forming the rosette of radical leaves are 

 larger, and lie flat on the ground. Still another interesting point in 

 leaf-adaptation is found in the nervature ; either the nerves are 

 parallel as in most monocotyledons, or they anastomose to form a 

 network as in most dicotyledons. In the former case the leaf is 

 simple, and of much stronger tissue ; the complicated net-veining is 

 found in wide-spreading leaves, and serves as a protection against 

 splitting. In the Fern group the simpler type of parallel nervature 

 prevails rather than the other, but during the Mesozoic age net- 

 veined fern leaves were very abundant up to the close of the Chalk 

 period. Then dicotyledons with leaves of a similar structure began 

 to appear, and seem gradually to have taken their place. 



Stahl closes the paper by a short examination of the leaf-tissues 

 as they are affected by environment. In the long, slanting or 

 bending leaves of monocotyledons the upright palisade cells of the 

 assimilating tissue are replaced by cells the longest axis of which is 

 parallel to the leaf-surface. By the massing of the strengthening 

 elements towards the centre of the tissue, between the upper and under 

 surface, the elasticity of the leaf is secured, and there is further 

 provision against bending strains in those leaves with parallel veins, 

 each of them being surrounded by strands of strong fibres. 



Annie Lorrain Smith. 



