I GENERAL CHARACTERISTICS 749 



grow throughout its Ufe, or so long as conditions are favorable. The actual rate 

 of growth may be slow, as in lichens growing on stones, or the Japanese dwarfed 

 oak trees a hundred years old and still in flowerpots, but it does not stop, and when 

 conditions are favorable it can always accelerate. Even the Sequoias of California, 

 or the Eucalyptus trees of Australia, 2000 years old and 300 or more feet in height, 

 continue to add a growth ring and to put out new twigs every year. Individual 

 organs such as leaves, flowers or the fruiting bodies of mushrooins and other fungi 

 have a determinate or finite growth, reaching a fixed size at the end of a given 

 growth period, like a mammal. However, these are not the whole plant, and the 

 stem and roots, or in the case of the fungi the mycelium spreading through soil 

 or host, grow continuously and indefinitely. 



Another characteristic of the pattern as a whole is the restriction of growth to 

 certain localized areas. The local regions in which cells are rapidly dividing are refer- 

 red to as meristems, and it is within, and particularly in the regions adjacent to, 

 these meristems that the main enlargement takes place. The stems and roots of 

 higher plants are long cylindrical organs, whose growth is at first essentially elonga- 

 tion. In the shoot, the meristem is at the apex, surrounded by young developing 

 leaf primordia, and as the cells in it divide they enlarge. They continue to divide 

 at first according to a characteristic pattern, but presently the cells furthest from 

 the center of the meristem stop dividing, and continue to enlarge or perhaps even 

 enlarge at an accelerated rate for a time. Most of this enlargement is along the 

 axis, resulting in an elongated cylindrical stem. Elongation in the root is very 

 similar, the meristem in this case being i to 2 mm behind the actual root tip, and 

 giving rise to new cells both in front and behind it. The cells which it produces at 

 the tip end become the root cap and are gradually sloughed off, while most of 

 the new cells are on the side remote from the tip and these elongate to form the 

 extending root. 



In the leaf, the meristem at first occupies the whole primordium but very soon 

 it is only the marginal cells which remain meristematic, first at the tip, and then 

 gradually more and more towards the base, the tip of the leaf generally ceasing 

 growth early. The meristematic cells in this case enlarge in two directions, to form 

 a sheet or plate of cells, and in addition there are a few divisions perpendicular to 

 the surface which give rise to the thickness of the leaf, perhaps a dozen or so cells 

 in the typical case (Avery, 1933). Growth of leaves is considered in more detail 

 in section VII (p. 797). 



In the algae and fungi a clearly defined meristem is not detectable, for the mass 

 of tissue formed is commonly considerably less, but the general pattern of small 

 cells dividing rapidly at or near the tip of the plant, and elongating for some time 

 after division to produce the cylindrical or perhaps plate-like structure, is similar. 

 Sometimes, however, in these plants only the nuclei may divide and the cyto- 

 plasm remain unified, enlarging to produce giant "cells" or coenocytes. This 

 occurs in the Phycomycetes (except where fruiting bodies are formed) and the 

 whole mycelium is a coenocyte with small nuclei throughout its length. Among 

 the green algae the Siphonales, of which Oeclogonium is a typical example, show 

 the same habit. JVitella, in a subgroup of the green algae, has been much used for 

 experiments on account of its very large cells, up to 10 cm long and 3 or 4 mm 



Literature p. 8:6 



