Structure and Function of Endodermis in Abietinece 291 
suberisation is not yet developed, is enclosed in a sheath of scale 
leaves, the cells of which are strongly sclerotic. 
It has been shown by Groom( 7 ) that transpiration in coniferous 
trees is not necessarily slow, and he found that the larch and beech 
showed the most rapid flow of the transpiration current, the Scots 
Pine, Pseudolarix and Abies succeeding them in this respect. The 
reason for this rapid loss of water by the trees lies in the fact that 
the aggregate leaf surface is very large, exceeding that of the ordinary 
dicotylous tree. Correlated with this, the coniferous leaf exhibits 
the phenomena termed by Prof. Groom “architectural xeromorphism.” 
Histological checks diminish the rate of flow of the transpiration 
current, and an important factor from this point of view is the 
suberisation of the radial and transverse endodermal walls. These 
serve as a barrier delaying the passage of the water stream from the 
central cylinder to the cortex. 
In conclusion I should like to express my thanks to Dr Eric 
Drabble, at whose suggestion this investigation was undertaken, and 
to whom I am indebted for helpful criticism during its earlier stages, 
and to Prof. Dame Helen Gwynne-Vaughan for help and advice 
during the latter part of its progress. The work was carried out at 
Birkbeck College, University of London. 
Summary 
1. The endodermis in the leaves of certain of the Abietineae has 
been shown to conform to a general plan, suberisation occurring 
always on the radial walls which are pitted, and on the transverse 
walls which are unpitted. The suberin is always deposited as a layer 
on each side of a lignocellulose core. 
2. The structure of the tangential walls varies in the different 
species described. They are often both lignified and suberised, but 
sometimes they, especially the inner tangential walls, consist of 
cellulose. Where these walls are suberised pits or breaks in the 
suberised membrane are present. 
3. In young leaves of Pinus sylvestris the development of the 
endodermal walls proceeds as follows: (1) cellulose, (2) lignocellulose, 
(3) lignocellulose with a surface layer of suberin. Thickening develops 
first in the radial walls. 
4. When the endodermal sheath is incomplete additional pro¬ 
tective tissues are often found. Thus the growing leaf base of Pinus 
is protected by sclerotic scales, and the basal region of the leaf in 
Picea and Abies by additional layers of lignified hypoderma. 
