110 PLANT STRUCTURES 
materials from the roots to the leaves, and manufac- 
tured products from the leaves to all growing parts: 
It is the former relation which has mainly determined 
the forms of stems. Even a very slender stem can 
convey a vast amount of water and food to a plant 
which is transpiring or growing actively, as we can 
test roughly by weighing a pot shrub as it begins to 
come into leaf, and again a week later, or comparing 
the growth of a pea with the size of its stem at the 
base. The surprising variation in length, thickness, 
form, position, and branching of stems is the plant’s 
response to external conditions—such as exposure, 
the competition of neighbouring plants, and so on— 
which resolve themselves ultimately into questions of 
wind-pressure, of temperature, of moisture, and in 
particular of light. The first duty of most stems is 
to spread out the leaves so that they may receive a 
maximum share of sunlight, and the complicated 
systerns of branches with which we are so well 
acquainted are devoted to this object, the leaves them- 
selves helping materially by the positions which they 
assume. This familiar and typical kind of stem, 
upright and column-like, beautifully constructed to 
bear the weight of leaves and branches, and to resist 
wind-pressure, alone furnishes a delightful study; but 
it can be dealt with only very briefly, as also some of 
the modifications which it undergoes under special 
circumstances. 
To plants which have not taken to a terrestrial 
existence, and which still inhabit their ancestral home 
in the water, the stem problem is comparatively 
simple. A flexible shaft capable of withstanding wave 
and current action suffices so far as mechanical con- 
