OBSHRVATIONS FOR YOUNG BOTANISTS 47 
If a growing aerial branch of mint be bent down and buried, 
it will be found that, after it has grown for some time under- 
ground, the microscopical details of the new part at once begin 
10 imitate the usual — but, of course, more decidedly marked — 
structure of an underground stem, showing that this structure 
solely depends upon the external conditions in which the stem 
grows. 
Aerial Stems. — Growing erect in air they must support 
themselves, and it is interesting to note all the different shapes 
of stems of herbs, to see how Nature strengthens them to 
prevent their falling. Thus, several, as the carrot, are fluted 
externally with stiffened ridges like buttresses ; others, though 
round, have a hardened tissue just beneath the surface and 
made from the pericycle, as in the wire-like flower-stem of an 
Ixia, &c., or, again, a fibrous layer is formed giving great 
flexibility, as in Hax. If it be a shrub or tree, then wood fibres 
are supplied in abundance. 
To prove how a stem will “ respond” to a strain the follow- 
ing experiment has been made ; the stem of a seedling sun- 
flower, which would have broken under a weight of 160 grins., 
bore a weight of 250 grms., after having been subjected for two 
days to the strain of a weight of 150 grms. The weight was 
subsequently increased to 400 grms. without injury. A simple 
experiment with leaf stalks of developing leaves in spring will 
show the same thing, as by suspending a given weight and 
subsequently comparing the tissues seen in cross sections in a 
normal leaf stalk, and in the one experimented on, the latter will 
have much more wood. From these experiments and observa- 
tions, a general law may be expressed by saying that just as the 
muscles on the limbs of animals become strengthened by use, so 
is it in plants. They “ feel ” any extra strain put upon them, 
and they “ respond ” accordingly by adding such tissues as are 
requisite to meet the demand. 
The principle upon which woody stems are rendered rigid is 
that of a combination of girders. A section of an iron girder is 
that of the letter I, and if a number of these are made to cross 
one another in the middle, such represents the arrangement of 
the woody bundles in a young stem. In place, however, of the 
connecting pieces of the I, there is a uniform mass of cellular 
tissue constituting the pith. In stems of a few years’ growth 
wedges replace the more isolated packets of wood. 
Aquatic Stems. — Tliese are supported by the water. Con- 
sequently there is no necessity to supply much “ mechanical 
tissue.” Instead of this, long air-chambers run up the stem 
which render it light and easily to be supported by the water. 
If a stalk of a water-lily flower, or other water-plant, be cut 
clean across and touched with Indian ink, beautiful impressions 
can be made of these air-chambers. If a plant usually growing 
on land be allowed to grow in water, the submerged parts im- 
mediately form air-chambers and cease to form much wood, 
