28 o 
Isabel Soar 
this leaf exhibits approximately the same degree of xeromorphy as 
does Pinus sylvestris, two or three layers of sclerenchymatous hypo- 
dermal tissue being present in this case also. 
III. Pinus excelsa Lam. 
The blue Himalayan Pine has leaves from 5-8 inches long growing 
in clusters of 2-5. The endodermal cells are rather wider than those 
of Pinus sylvestris, but are shorter when seen in longitudinal section. 
Their walls are also thinner. 
The radial walls have a central lignified core which is suberised 
on either side and pitted. On treatment with strong sulphuric acid 
the outer tangential walls become fully carbonised after a time, while 
the inner tangential walls are only carbonised in certain places. 
There is less lignification of both tangential walls in this type than 
is the case with either of the two preceding species, and only those 
parts of the inner tangential wall are lignified which abut on lignified 
pericyclic tissue. The transverse walls are lignified, suberised and 
unpitted. 
This leaf is on the whole less xeromorphic in character than that 
of Pinus sylvestris as regards the delicacy of the endodermal cells, 
the infrequent lignification of the inner tangential wall, and the 
presence generally of a single layer only of lignified hypodermal 
tissue. No breaks were found in the continuity of the endodermal 
sheath in this or in any of the species of Pinus investigated. 
IV. Pinus strobus L. 
In the Weymouth Pine the cells of the endodermis are not so 
regular as is the case in the preceding species and a further reduction 
in the amount of thickening present is noticeable. The radial and 
transverse walls have a core of lignified material suberised on either 
side. Only the former walls are pitted. The outer tangential walls are 
of lignocellulose, but the inner tangential walls are usually quite 
unlignified, at most a very slight layer of thickening appearing in a 
few cells. All the cell walls are more delicate than those of Pinus 
excelsa. A single layer of lignified hypodermal tissue is present. No 
passage cells were observed in the endodermis. 
V. Picea excelsa Link. 
Daguillon has described the cotyledon of Picea and pointed out 
that, as in Pinus, there is a distinct endodermis, but that of Picea 
is distinguished by having bands of thickening on the radial walls. 
The pericyclic tissue has some thickened elements and some with 
