49, THE OAK 
its next neighbour below, and if a line be drawn from 
the insertion of any one leaf through the points of in- 
sertion of those above, it will describe a spiral, and will 
eventually come to a leaf standing directly above the 
leaf started from. In doing this the spiral line will 
pass twice round the stem, and through the points of 
insertion of five leaves. This is shortly expressed by 2. 
The previously homogeneous embryonic tissue in 
the section now shows certain patches of greyer, closer 
tissue, arranged round the centre in a peculiar manner; 
these are transverse sections of the young vascular 
bundles—strands which at present are distinguished 
chiefly by the small diameter of their cells, whence the 
darker grey appearance. 
These strands when young are called procambium 
strands. Their cells are distinguished from the other 
embryonic cells around by growing more in length and 
dividing less frequently across their length, and by 
erowing less in breadth and dividing more often by 
longitudinal walls. 
On transverse sections a little lower down there may 
be seen a number of elongated and curved patches of 
procambium, as shown in fig. 9. On the section it will | 
be noticed that the larger strands are so arranged that 
they enclose a five-angled mass of central tissue (the 
pith), the five corners pointing to the angles of the 
young stem to which the leaves are attached. At the 
corners or ends of the rays just referred to are in some 
cases two or three smaller strands. 
