34 A TEXT-BOOK OF BOTANY 
there is no regular annual fall of leaves. Such leaves en- 
dure for a varying length of time; but as there is no regular 
period for all of them, the shrub or tree always appears in 
foliage. In the temperate regions the most conspicuous 
evergreens are the pines and 
their allies. A comparison 
between the needle-leaf of a 
pine and the leaf of an ordi- 
nary deciduous tree will show 
what the evergreen habit in- 
volves in temperate regions 
(Fig. 30). The leaf of a pine 
must be protected so as to 
endure the winter, and this 
Fic. 31.—Cross-section of the needle- 
leaf of a pine, showing the epider- hag involved reduction of 
mis with heavy cuticle (e), in which 2 
are sunken stomata (s); masses of surface and extremely thick 
heavy-walled cells (h) beneath the protective layers about the 
epidermis; the mesophyll region (m) S 
containing chloroplasts, and the cen- mesophyll (Fig. 31). This 
ee has diminished the ability to 
work; but it has saved the 
tree the necessity of putting out a complete new crop of 
leaves for the next season. The deciduous leaf, on the 
other hand, is broad and thin, with great capacity for 
work; but this forbids protection during the winter. 
20. Special forms of leaves.— Besides the ordinary leaves 
that have been considered, and which are called in distinc- 
tion foltage leaves, there are special forms of leaves whose 
chief work is different. In so far as they are green, they 
manufacture carbohydrates as do the fohage leaves, but a 
distinct change in structure and behavior indicates that this 
is not their chief work. 
(1) Scales—The most conspicuous illustrations of 
leaves that have become modified into scales are to be found 
in subterranean stems and scaly buds. Underground stems 
cannot produce foliage leaves on account of the absence of 
