636 



ECOLOGY 



to tide over long unfavorable periods; and, finally, the high concentra- 

 tion of the cell sap, which makes water retention possible in spite of 

 prolonged exposure to conditions that cause desiccation in plants with 

 dilute cell sap. 



10. MISCELLANEOUS LEAF STRUCTURES AND 

 RELATIONS 



Leaves as organs of run-off. — As previously noted, hairy coats and surface ex- 

 cretions often are of value, especially in water plants, in preventing the wetting 

 of leaf surfaces, thus facilitating an undisturbed ex- 

 change of gases through the stomata. In the rainy 

 tropics, where storms occur daily and where the air is 



Fig. 931. — A leaf 

 of Ficus religiosa^ a 

 tree of the tropical rain 

 forest, showing a so- 

 called dripping point 

 (d). — After Stahl. 



Fig. 932. — A portion of a leaf of a tropical Asplenium^ 

 illustrating reproduction by leaves; on the under sides of 

 the ultimate leaf divisions (pinnules) are fruit dots or sori 

 (i), in which are sporangia with their spores ; on the upper 

 sides of the pinnules are bulbils (6), which here have germi- 

 nated while connected with the parent plant, giving rise to 

 bulblings whose leaves (/) already are conspicuous. 



almost constantly humid, many leaves have long attenuated tips, sometimes known 

 as dripping points or gutter points, which are supposed to facilitate the run-off 

 of precipitated water that might otherwise impede stomatal activity (fig. 931). 

 Grooved and channeled petioles may act similarly. It has been suggested that 

 there is a sort of correlation between run-off and root direction, horizontal roots 

 characterizing plants with leaf drip, and vertical roots those with petiole drip ; while 

 this is unhkely, the direction of root growth might be thus determined if run-off 

 were the sole source of water, since roots are prohydrotropic. 



Leaves as reproductive organs. — Ferns. — In the ferns the foliage leaves com- 

 monly bear spor-ngia, which are grouped in brownish fruit dots {sari) on the 

 back of the leaf (as in Aspidium, figs. 1128, 1129), or are borne under the 

 recurved leaf margins (as in Pteris). In some ferns (as Osmunda) the sporangia are 

 borne on special reproductive branches, or even on special leaves which contrjist 

 strongly with the foliage leaves. Fern leaves may take part also in vegetative 

 reproduction. Cystopteris bulbifera and species of Asplenium develop leaf bulbils, 



