1921] HOLM—CARYA AND JUGLANS 383 
This tissue does not contain starch or crystals, but is interrupted, 
here and there, by narrow strands of secondary leptome, covered 
by young thin-walled stereome (fig. 16, St), in two concentric 
bands. Then follows a continuous zone of cambium connecting 
the four collateral mestome strands, and from which (the cambium) 
some few young, wide, porous vessels have become developed. 
Beside this secondary mestome the protohadrome vessels are very 
distinct, forming four short narrow rays of annular and spiral 
vessels, 
The very commencement of the formation of these secondary 
tissues, the cortex and the collateral mestome strands, but not the 
cork, can only be traced at the youngest, the apical, portion of this 
toot. The earliest appearance of the secondary formations depends 
upon a double meristem arising along the inner flank of the primary 
leptome, and from which secondary leptome and hadrome become 
ormed. Outside the protohadrome the pericambium then com- 
mences to divide, forming another meristem, which in Juglans gives 
tise first to parenchyma, a secondary cortex, and a little later to 
a peripheral cork. Regarding the stereome, so amply represented in 
the secondary cortex, this tissue is totally absent from the primary 
structure of this root. It arises outside the leptome (fig. 16, S?), 
and is formed by the secondary cortex, soon developing to distinct 
Separate strands, arranged in one or several more or less concentric 
ands. 
In old, thick, lateral roots the epidermis and the primary cortex 
are replaced by many layers of thin-walled, homogeneous cork, which 
surround a broad zone of compact thin-walled parenchyma (second- 
ary cortex), the cells of which contain much starch and numerous 
aggregated crystals of calcium oxalate. In this secondary cortex 
are five or six concentric bands of isolated stereome strands (fig. 17, 
St). Viewed in longitudinal sections these stereome strands 
traverse the parenchyma in wavy, not parallel lines. The stele 
contains a peripheral zone of almost continuous leptome, also 
several strata of cambium, beside a dense mass of hadrome, in 
which wide porous tracheids with bordered pits are quite con- 
spicuous. Thick-walled libriform, and thin-walled parenchyma 
with starch represent also a large part of the stele. The medullary 
