lOS PLANT PHYSIOLOGY AND ECOLOGY 



Embed and cut on the microtome. Study the origin and structure of 

 the meristematic regions Cut cross-sections of a woody stem for the 

 study of cambium, and the cork meristem or pliellogen. Cut cross- 

 sections of a flower bud and note the meristem. 



130. Conditions that influence growth. In most one-celled 

 plants, growth takes place equally in all directions. In fila- 

 mentous and massive forms, growth is greatest in one direction, 

 and division is correspondingly modified. The cell and the plant 

 become more or less drawn out, as though influenced by two 

 poles. Although this phenomenon, which is called polarity, is 

 all but universal among plants, its cause is obscure. The polarity 

 so characteristic of stemmed plants is largely a matter of the 

 control exerted by gravity, but in the case of thallophytes, e.g., 

 liverworts, lichens, and algse, other factors, light, water, etc., enter 

 in. Gravity is the most important of all forces in determining 

 the direction of growth. Broadly speaking, the main axis of 

 vascular plants is in line with the pull exerted by gravity, while 

 the secondary axes, branches, leaves, etc., are more or less at 

 right angles to it. The growth of the root is with the pull due 

 to gravity, that of the shoot against it. The relation of growth 

 to gravity will be further discussed under geotropism. 



131. The amount and rate of growth are determined by the 

 physical factors of the habitat. Growth is directly affected by 

 the condition of the plant, especially its turgidity and nutrition, 

 but these are under the control of the physical factors. The sup- 

 ply of oxygen is an important factor, though its effect is observ- 

 able only when the usually adequate supply is greatly reduced, 

 as in the case of many water plants and humus plants, particu- 

 larly certain saprophytes. Water is, for many reasons, of the first 

 importance in growth. It is necessary for the mechanical stretch- 

 ing of the cellulose wall, which is a prerequisite for the growth 

 of all tissues. It is not only necessary for the formation of car- 

 bohydrates, but also for the absorption of nitrogen compounds 

 and other necessary salts. It serves as a vehicle for the trans- 

 location of elaborated foods, and is indispensable in maintaining 

 the turgidity of the plant. Temperature is equally important in 

 growth. Apart from the indirect effect which it exerts through 

 its influence upon absorption, photosynthesis, etc., it controls 

 growth directly through digestion, assimilation, and respiration. 

 Indeed, heat and water may be termed the two requisites for 



