6 44 



ECOLOGY 



as is shown by the fact that some of the inner scales are tipped by a minute leaf 

 blade (fig. 1 1 60). When such a bud germinates, the outer scales drop off, while the 

 inner scales progressively assume more and more the characters of foliage leaves, in 

 color and persistence, as well as in shape and size. In Viburnum Lentago the bud 

 is protected by two large scales with long attenuated tips, 

 which in spring enlarge at the end into small green blades 

 (figs. 949, 950). Not only do such facts show clearly the 

 essential morphological equivalence of foliage leaves and 

 scale leaves, but the identical possibilities of their pri- 

 mordia are capable of experimental demonstration. The 

 removal of the leaves from a pine or lilac shoot while the 

 buds are forming is followed by the development into 

 foliage leaves of primordia that otherwise would become 

 scale leaves, and in most plants the removal of the termi- 

 nal bud during development is followed by the develop- 

 ment into shoots of lateral buds which otherwise would 

 have remained as primordia. Probably, therefore, the 

 stimuli which determine whether primordia develop into 

 bud scales or into foliage leaves are external, but the pre- 

 cise factors involved are unknown. Among the features 

 of buds most in need of explanation are these: the 

 arrest of the shoot primordia at a certain definite stage 

 in development, apparently without external inhibitory 

 influence; the failure of the external leaf primordia 

 to develop into foliage leaves; and the development in 

 unusual thickness of cutin or cork layers on the ex- 

 posed under surfaces of the scale leaves. Experiment 

 has thrown some light on the cause of arrested shoot 

 development, as will appear elsewhere (p. 735), but why 

 leaf primordia that apparently are exposed to favorable 

 conditions fail to reach their developmental possibilities 

 remains to be explained. Nor is it understood why scales 

 develop cutin in such great amount, when foliage leaves 

 growing under apparently similar conditions exhibit rel- 

 atively slight cutinization ; sometimes (as in Tilid) the 

 bud scales have cork as well as cutin, while the foliage leaves have cutin only. It is 

 possible that the relatively high transpiration of late summer is here a factor of 

 importance. Aerial scale leaves, other than those of winter buds,, occur in many 

 inflorescences (fig. 947), in various plants without chlorophyll (as Monotropa, fig. 

 1104), and even in some green plants (as Asparagus and Equisetum, figs. 1054, 

 1055) in which the stems are the chief synthetic organs. 



FIGS. 949, 95- 



Shoots of the sweet vi- 

 burnum (Viburnum Len- 

 tago) : 949, a shoot as seen 

 in late winter, showing 

 the two prominent scale 

 leaves (s), which enclose 

 the bud; 950, the same 

 shoot in early spring, 

 showing the leaf blades 

 (/) which have developed 

 through the renewal of 

 growth in the bud scales 

 (s of fig. 949) and also 

 the leaves (/') which have 

 developed from the bud 

 within the scale leaves; 

 note the leaf scars of the 

 previous season (r). 



