642 



ECOLOGY 



branous sheaths surrounding the stem in the Polygonaceae (fig. 822), while they 

 are coherent with the petiole in the roses (fig. 1094). Stipules may persist 

 through the life of the leaf or they may be caducous (i.e. falling as the buds open, 

 fig. 948); usually in both cases they are precocious in their development, thus 

 affording some protection to the rest of the develop- 

 ing leaf or shoot from transpiration and other detri- 

 mental factors. Probably this is the only role of most 

 caducous stipules, and it is well illustrated by the large 

 stipule of Ficus, which encloses the developing blade, 

 falling as the latter expands (fig. 714). Many cadu- 

 cous stipules are small and 

 have no apparent role. 

 Precocious persistent stip- 

 ules also may afford pro- 

 tection, as in the peas 

 (figs. 939, 943), where they 

 stand close together verti- 

 cally, with an undeveloped 

 leaf or shoot enclosed be- 

 tween them; in Lathyrus 

 ochroleucus, frosts may kill 

 young parts protruding 

 from the stipules, while not 

 injuring the enclosed parts. 

 The chief role of persistent 

 stipules, which, in contrast 

 with caducous stipules, are 

 almost always green, is the 

 manufacture of foods; in 

 plants with large stipules, 

 as in the peas, this role 

 assumes quantitative im- 

 portance, and in Lathyrus 



Aphaca, the stipules much surpass the blades in size 

 and in synthetic capacity. If the developing blades of 

 Prunus serotina are cut away, the stipules grow to a 

 considerable size and live for a longer time than usual ; 

 doubtless they manufacture more food than under 

 ordinary conditions. 



946 



FIGS. 945, 946. Cross 

 sections of leaves of a loose- 

 strife (Lysimachia): 945, a 

 section through a scale leaf, 

 showing a relatively undif- 

 ferentiated mesophyll (m), 

 resembling sponge tissue, and 

 the prominently cutinized 

 lower (outer) epidermis (e); 

 946, a section through a foli- 

 age leaf, showing a row of 

 conspicuous palisade cells 

 (/>), a broken row of shorter 

 palisade cells (p f ), and the 

 sponge tissue (s); figs. 945 

 and 946 highly and equally 

 magnified. 



FIG. 947. An inflor- 

 escence bud of the arti- 

 choke (Cynara Scolymus\ 

 showing the overlapping 

 scale leaves (forming an 

 involucre), which protect 

 the delicate flower buds 

 within; the leaves are ar- 

 ranged in many ortho- 

 stichies, illustrating a phyl- 

 lotactic system of high 

 rank. 



Scale leaves. General features. Scale leaves contain little or no chlorophyll, 

 and hence are not foliage organs; usually they are small yellowish or brownish struc- 

 tures attached to the stem by a broad base, and without prominent veins or leaf 

 teeth; the mesophyll commonly remains undifferentiated through life. 



Subterranean scale leaves. Underground stems, at least when young, are clothed 

 more or less thickly with scale leaves. In bulbs the closely imbricated scale leaves are 

 much thickened, making up the main body of the organ; obviously the role of such 

 scale leaves is the accumulation of surplus food and water, which commonly are 



