LEAVES 



643 



s'- 



utilized during subsequent development (fig. 991). The scale leaves of rhizomes 

 are much smaller and thinner than bulb scales, and often are more ephemeral, ap- 

 pearing to have no role of importance, except where they protect the growing stem 

 apex as it pushes through the soil (as in Spartina, figs. 979, 980). 



In many plants with both aerial and subterranean stems (as Lysimachia, fig. 944) 

 there are all gradations between scale leaves and foliage leaves, the former 

 having undifferentiated colorless mesophyll, and a 

 strongly cutinized lower epidermis, and the latter having 

 green palisade and sponge cells (figs. 945, 946). Both 

 kinds of leaves have similar positions and arise from 

 similar primordia. Furthermore, in many cases, foliage 

 leaves develop from scale primordia when exposed from 

 the outset to light and air, and scale leaves may develop 

 in the soil from the primordia of foliage leaves. Thus 

 the distinction between such leaves is not inherent, but 

 a matter of relation to external conditions, though the 

 precise factors involved are imperfectly known. 



Bud scales. In most trees and shrubs of cold and 

 arid climates, buds are formed in the growing season 

 previous to their full development ; after reaching a 

 certain size, they remain for some months in compara- 

 tive quiescence. In most cases the outermost leaf 

 primordia attain their full development the first season, 

 becoming hard and thick scales (figs. 952, 953, 1057- 

 1059); on the other hand, the innermost primordia 

 beneath the closely imbricated outer scales are incom- 

 pletely or not at all developed until the following season, 

 when they grow into foliage leaves. Bud scales protect 

 the embryonic shoot by reducing transpiration and by 

 minimizing the effect of sudden temperature changes, 1 

 their thick cutin or cork layer often being supple- 

 mented by an external resin coat (as in the cottonwood) 

 or by internal hairs or by both combined (as in the horse 

 chestnut) ; the scales also are beneficial in protecting 

 the delicate inner portion of the bud from mechanical 

 injuries. The protective efficiency of bud scales is 

 shown by the injury done to germinating buds by a spring frost that would have 

 been harmless if occurring before the shoot had emerged from the scales, though part 

 of the harm is due to the fact that germinating buds contain much more water than 

 do the resting buds, and hence are more subject than the latter to injury through 

 frost. The winter buds of Viburnum lantanoides and Cornus sanguinea are without 

 scales, the buds of the latter being protected by a dense growth of hairs. 



Bud scales, like subterranean scales, grade into foliage leaves (fig. 948). In the 

 horse chestnut and in various maples the scales represent the basal portion of the leaf, 



1 The importance of bud scales in protecting from low temperatures often is overesti- 

 mated; in the winter, ice forms abundantly in the bud tissues. 



FIG. 948. A develop- 

 ing shoot of the choke 

 cherry(Prunus virginiana), 

 showing a gradual tran- 

 sition from the outer scale 

 leaves (s) of the winter 

 buds which are shed early, 

 through the inner scale 

 leaves (s 1 ) which elongate 

 as the bud opens, to the 

 ordinary foliage leaves (I) ; 

 note also the slender sti- 

 pules (st). 



