182 THE GROWTH OF ROOTS AND STEMS. 



cork-cells, as, for example, in the ordinary cork of com- 

 merce. 



The lenticels are closed in winter by the formation of 

 ordinary compact cork-tissue. There are air-spaces between 

 the cells of the cortex and of the medullary rays, so that all 

 the living tissues are aerated by way of the lenticels. 



216. Practical Work on Woody Stems. 



(a) Cut sections of twigs of Elder, Willow, Elm, Oak, or other trees, 

 and notice in the youngest parts the separate bundles arranged in a 

 ring. In the older parts note that the continuous 'cambium has pro- 

 duced a solid mass of wood on its inner side, while the surface of the 

 stem has become covered by a brown coating of cork. Peel off the cork, 

 and notice the green cortex, then a zone of colourless tissue (bast), 

 which is separated from the hard wood by a moist sticky layer (the 

 cambium). Cut sections and stain some with aniline chloride, others 

 with iodine. In the wood, notice the rays and the "rings." Look for 

 trees showing stages in healing of wounds where branches have broken 

 off. Compare the wood of various common trees ; make a collection of 

 polished slices. Find out all you can about timbers density ; hard- 

 ness ; uses; "silver-grain"; "burrs"; "shakes"; canker, dry rot, 

 and other diseases, etc. 



(b) Next cut a thin slice out of the thickest part of a branch with a 

 saw, and rub down one of the cut surfaces on fine glass-paper until it is 

 very smooth. Examine it with the aid of a lens, and note the parts as 

 indicated in the preceding paragraphs (Arts. 207-215). Examine logs 

 of wood showing the cut surfaces, or the cut stumps of trees. 



(c) Strip the cork-layer from twigs of Elder and other plants which 

 show lenticels plainly on the surface, and notice that lenticels are not 

 merely surface-marks, but that they pass right through the cork to 

 the green cortex- tissue. Strip off, layer after layer, the bark of a 

 Birch twig, and notice that the lenticels go through all the layers. 



(d) Dip a twig of Elder, Birch (or other plant which shows lenticels 

 plainly), into boiling water, and notice the air-bubbles which escape 

 from the lenticels. This shows that the cortex of the stem has air- 

 spaces, and that gases can pass through the loose lenticel tissue. 



217. Forms of Stem. In some plants the aerial portion 

 of the stem is very short, so that the leaves seem to spring 

 from the top of the root, while arising from the centre of the 

 tuft of leaves is the upright flowering axis. Such leaves are 

 often said to be radical, and the condensed portion of the 

 stem, together with the upper part of the root, is known 

 popularly as the " root- stock." Very commonly this is 



