98 PLANT LIFE 
by which they can be very greatly lengthened, 
although they become, of course, considerably 
thinner as the result. This tissue is often 
called collenchyma, from the peculiarly bright 
gelatinous appearance of the walls. It is 
specially adapted, by its extensibility, to the 
requirements of small and growing organs, 
whilst its inferior value as a supporting tissue 
is largely compensated by its advantageous 
position in the stem. Indeed, collenchyma 
affords a wonderful example of an accurate 
balance of qualities possessed by a tissue 
which is required to be carefully adjusted to 
meet very diverse needs. For whilst the 
function of support is its main raison @étre, 
it is obvious that it must not be so strong 
or so rigid as to materially interfere with the 
growth in length of the organ in which it is 
present. 
It sometimes happens that the structures 
on which the rigidity of a stem depends have 
to be provided in a rather different way. In 
wheat, and most other grasses, the stem 
continues for some time to elongate just above 
the node or “‘ knot.”’ Most people know that 
it is easy to pull the stem out from the knot, 
and that the broken end is soft and succulent. 
But a series of such weak joints in a stem, 
however well the mechanical requirements 
might be fulfilled in the intervening regions, 
would of course be fatal to the retention of 
an erect position. In the grass this weakness 
is remedied by a curious arrangement of the 
