BOOK II. 
GENERAL CONSIDERATIONS. 
279 
internode of the latter ceases to lengthen, the leaf has actually 
arrived at its complete formation. When fully grown it re- 
peats in a much more perfect manner the functions previously 
performed by the cotyledons : it aerates the sap that it re- 
ceives, and returns the superfluous portion of it downwards 
through the bark to the root; it also sends tubular tissue down 
between the medullary sheath and the bark, thus forming the 
first ligneous stratum, a part of which is incorporated with 
the bark, the remainder forming wood. 
During these operations, while the plumule is ascending, 
its leaf forming and acting, and the woody matter created by 
it descending, the cellular tissue of the stem is forming, and 
expanding horizontally, to make room for the new matter 
forced into it ; so that developement is going on simultaneously 
both in a horizontal and perpendicular direction. This pro- 
cess may not inaptly be compared to that of weaving, the 
warp being the perpendicular, and the weft the horizontal, 
formation. In order to enable the leaf to perform its func- 
tions of aeration completely, it is traversed by veins originating 
in the medullary sheath, and has delicate pores (stomates), 
which communicate with a highly complex pneumatic system 
extending to almost every part of the plant. 
Simultaneously with the descent of woody matter down- 
wards from the leaf, the emission of young roots, and their 
increase by addition to the cellular substance of their points, 
take place. They thus are made to bear something like a 
definite proportion to the leaves they have to support, and with 
which they must of necessity be in direct communication. 
After the production of its first leaf by the plumule, others 
successively appear in a spiral direction around the axis 
at its growing point, all constructed alike, connected with 
the stem or axis in the same manner, and performing pre- 
cisely the same functions as have been just described. At 
last the axis ceases to lengthen ; the old leaves gradually fall 
off ; the new leaves, instead of expanding after their formation, 
retain their rudimentary condition, harden, and fold over one 
another, so as to be a protection to the delicate point of 
growth ; or, in other words, become the scales of a bud. We 
have now a shoot with a woody axis, and a distinct pith and 
T 4 
