116 



BOTANY 



PAET I 



usually yellowish or brown, more simple in their structure than the 

 leaf-blade, and are soon shed. 



The two stipules in such cases are frequently more or less completely united to 

 form a single structure standing in the axil of the leaf. They may also surround 

 the stem and form a closed tube which encloses the younger leaves of the bud. 

 This is the case in the India Rubber plant (Ficus elastica) which is frequently 



grown in dwelling-houses ; in this the sheaths 

 are broken off at their bases and carried up on 

 the next younger leaf as it unfolds. In the 

 Polygonaceae they are broken through and 



7i / remain as dry sheaths (ochrea, Fig. 617) sur- 



rounding the stem. 



When the stipules take part in the 

 assimilation of carbon dioxide they are 

 green and resemble the leaf-blade in 

 structure (Fig. 209). 



In some species of Galium in which the 

 stipules completely resemble the leaf- blades, 

 there is an appearance of whorls of four, 

 six, or eight leaves ; in reality the arrange- 

 ment of the two leaves is decussate, each leaf 

 having one or more pairs of stipules according 

 to the species. Only the two leaves have buds 

 in their axils. 



The leaf-base may form a sheath ; 

 this is more commonly the case in 

 Monocotyledons than in Dicotyledons. 

 In the Grasses (Fig. 133 v) the sheath 

 is split along one side, but in the 

 Cyperaceae it is closed. The sheath of 



FIG. 133. Part of stem and leaf of a grass, the grass leaf, which encloses andsilp- 

 h, Haulm ; v, leaf-sheath ; fc, swelling p 0r ts the lower delicate portion of the 

 of the leaf-sheath above the node; s. , . n , j ,. 



partofieaf-biade; z, iiguie. (Nat. size. stl11 growing uitemode, continues at the 

 After SCHENCK.) base of the sessile lamina into a mem- 



branous outgrowth called the Hgula (/) ; 



at its base immediately above the node the sheath is swollen 

 (Fig. 133 A). 



Anisophylly and Heterophylly. Some plants bear diversely- 

 formed foliage leaves either in different zones of the stem (HETERO- 

 PHYLLY, Fig. 135) or in the same zone, but on the two sides of the 

 shoot which thus becomes dorsiventral (ANISOPHYLLY, Fig. 134). 

 Asymmetry of the leaves is often associated with anisophylly. Many 

 water-plants exhibit heterophylly, having ribbon -shaped or liighly- 

 divided submerged water-leaves adapted to life in water and less 

 divided stalked aerial leaves (Fig. 135). The leaves which the Ivy 



