Isabel Soar 
278 
About this stage lignified sclerenchymatous elements appear be¬ 
tween the two bundles of each of the needles (Fig. 5, d ). These ele¬ 
ments increase in number, spreading towards the centre of the stem, 
and finally occupy the entire space between the two vascular bundles 
and extend round these on either side. At the same time cleavage 
into two proceeds, the tissues of the leaves becoming more or less 
completely separated (Fig. 5, e). Differentiated pericyclic cells then 
appear on the abaxial side of the sclerenchymatous tissue between 
the bundles. These are recognisable as lignified tracheidal tissues 
with young bordered pits. A row of a few regularly arranged cells 
with dense contents is now formed outside the developing pericyclic 
tissue. This takes at first the form of four or five cells in the region 
between the bundles (Fig. 5, e), but these rapidly increase in number 
and spread round the bundles, following the course of the pericyclic 
tissue previously developed (Fig. 5,/). These endodermal cells have 
most of their radial longitudinal walls lignified when they first be¬ 
come recognisable and suberisation of these walls soon follows. A 
slight amount of lignification is seen in both the inner and outer 
tangential walls. Details of the arrangement of the leaf tissues at 
this stage are given in Fig. 6, which shows the leaf in transverse 
section. 
As the development of the endodermis proceeds, the scleren¬ 
chymatous tissue between and around the bundles diminishes in 
extent and finally disappears. The position in which the endodermis 
develops last in passing up into the leaf is on the flat adaxial side of 
this structure, where it is in contact with the other needle of the pair. 
It is interesting to find that no completely thickened endodermal 
layer is present within the sheath of sclerotic scale leaves investing 
the base of the needles. These scale leaves afford protection for the 
growing base of the leaf which possesses no thickened hypodermal 
tissue. 
The general structure of the endodermal cell in the leaf of Pinus 
sylvestris shows that these cells must check to a certain extent the 
passage of water from the xylem to the outer tissues of the leaf. The 
suberisation of the radial and the transverse walls of the cells prob¬ 
ably retards the passage of water outwards through the endodermal 
walls, only the lignocellulose strip in the centre of the wall being 
permeable. The transpiration current must thus enter the endodermal 
cell chiefly by way of the inner tangential wall and pass through the 
protoplasm to the outer tangential wall. Further, passage of water 
through the tangential walls is restricted owing to the partial suberisa- 
