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, i.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 trachea?, 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 (Anackaris or Elodea), 

 found abundantly in all our rivers, canals, &c. 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- 



