THE STEM. 43 
“fall of the leaf” is not, as might be imagined, a great waste of 
protoplasm. All the contents of the leaf cells are, in fact, with- 
drawn into the stem before this takes place. The fall itself is 
effected by formation of a layer of cork running right across the 
insertion of the leaf-stalk, z.e., the point where it joins the stem. 
Separation now readily occurs, leaving, not a raw surface, but a 
neat ‘‘ scar’’ covered by cork. 
The protoplasmic parts of the stem carry on respiration, as we 
have seen to be the case in the root (p. 18). The circulation of 
gases mainly takes place in the cavities of the air tubes and inter- 
cellular spaces of the ground-tissue. In aquatic stems these last 
form air chambers of considerable size. In terrestrial stems com- 
munication between the intercellular spaces and the exterior is 
kept up by the stomata and lenticels (see further p. 65). 
The stem, or more correctly speaking the vegetative shoot, may 
subserve the function of reproduction, which in this case is termed 
vegetative. Reproductive organs proper are specially modified for 
the performance of their function, and are not concerned with 
nutrition. Vegetative organs, on the other hand, have mainly to 
do with nutrition, and, if they reproduce, are either not modified 
at all for that purpose, or at any rate not so profoundly as to 
interfere with their chief use. Vegetative reproduction depends 
upon the power of the stem to produce adventitious roots. One 
of the simplest cases is where branches are liberated by the rotting 
of the main stem, and become fresh plants. A notable example 
is the common aquatic weed, water-thyme (Anacharis or Elodea), 
found abundantly in all our rivers, canals, &e. This plant is a 
native of North America, and was introduced into Ireland about 
1836, and into England about 1841. Reproducing solely in the 
way described, it soon became a serious nuisance, even hindering 
canal navigation. Attempts were made to destroy it by means 
of cutting implements, but as every little bit cut off became a 
new plant, its increase was only augmented. Somewhat similar 
to this is the case of creeping stems, each node of which can 
develop roots and send up a shoot. Rotting or severance of the 
internodes makes the new individuals quite independent of the 
parent plant. The same sort of thing occurs with horizontal 
underground stems. Stolons and suckers are branches specially 
adapted for vegetative reproduction, and any strawberry bed will 
show how well the former are able to produce new plants. Cut- 
tings (see p. 13) form an artificial means of propagation akin to 
the preceding. ‘‘ Layering,” in which branches are fastened to 
the ground and induced to form adventitious roots, is of the same 
nature. Grafting is practically the planting of a shoot in an 
incision made in another stem instead of in the ground. Vegeta- 
