' 
1896 | A RUST AND LEAF CASTING OF PINE LEAVES 435 
color, those affected by the Coleosporium may be readily detected 
for perhaps 100™ or more by the pale yellowish hue of the 
leaves and the general thinness of growth. An examination of 
the affected trees shows that the yellowish color is due to the 
effects of the fungus, which is confined almost entirely to or near 
the tips of the needles formed the previous season. The thin 
appearance is owing largely to lack of leaves, those on the tree 
being mainly of the previous year’s growth, all the others having 
prematurely fallen. 
The changes in the host and parasite which precede the 
effects aoted will now be discussed, attention being called first 
to some of the anatomical and physiological questions connected 
with the normal leaves, in order that what is said in regard to 
the effects produced by the fungus may be better understood. 
ANATOMY OF THE NORMAL MATURE LEAF. 
The leaves of Pinus Virgintana are asa rule borne in pairs. 
They are of a bottle-green color,‘ vary in length from 2.5 to7.5™) 
et nearly semicircular in outline, the dorsal side being 
“urved and the ventral nearly flat. Cross sections of numerous 
Haves from trees grown under varying conditions show that the 
poles are nearly the same size throughout, namely, from 0.60 
> 0.62™" in diameter. Transverse sections of the leaf show that 
tis divided into three well-defined regions, namely, cortical, 
mSophyll, and fibro-vascular (fig. x5 the cortical region 
aha ve periphery, and varies in thickness and structure 
bins to the part of the leaf under observation ; the part of 
tree, with respect to sun and shade, from which the leaf is 
aken; and the condition of the soil, as regards moisture, where 
the tree is grown, | 
a cortical region is separable into three parts : (1) the 
“Pidermis, consisting of thick-walled cells quite regular in size 
x Ripceway, Nomenclature of colors, 1886. 
ae Barnes, and COULTER: Handbook of oge = 
a, te: alae of the North American pines based up 
» 250. 1886, 
gsection, 1886; also 
leaf anatomy, Bot. 
