THE ORGANIZATION OF THE INDIVIDUAL PLANT 139 



in order to support the requisite leaf spread. A digestive system is un- 

 necessary, since the food is manufactured within the cells and is either 

 used at once or stored in simple, easily altered forms. A transporting 

 system is, however, essential for carrying water, food and inorganic salts, 

 and products of metabolism to different parts of the plant body. The chief 

 by-products of plant metabolism, oxygen from photosynthesis and carbon 

 dioxide and water from food oxidation, are themselves substances neces- 

 sary to the plant and are more or less completely reutilized. Oxygen and 

 carbon dioxide are gases that can diffuse through the leaf surfaces, and 

 since few liquid or solid wastes are produced the plant needs no special 

 excretory system. Coordination of the activities of the various parts of 

 the plant is simple and largely under direct chemicophysical control; the 

 plant has developed no complex nervous system or highly specialized 

 sense organs. 



The Indeterminate Scheme of Plant Growth. A second major difference 

 between the higher plants and the metazoan animals is the method of 

 growth. In the dicotyledons, zones of active growth lie everywhere 

 beneath the surface — at the ends of the branches, at the root tips, and 

 forming a sheath, the cambium, around the branches, stem, and roots. 

 The cells of these zones remain always young, unspecialized, and similar 

 in characteristics and potentialities to those of the embryonic plant. 

 Together these growth zones constitute the meristem. Throughout the 

 active life of the plant, the cells of the meristem continue to reproduce, 

 giving rise to new tissues and causing the plant to increase in size. The 

 cell layers thus produced are added to those previously present, and the 

 individual cells cut off from the meristem become specialized for particu- 

 lar functions according to their location in the plant. 



Many tropical plants grow throughout the year; in these, the meristem 

 is active until the death of the plant. More often, especially in temperate 

 regions, growth takes place chiefly during the spring and summer; the 

 meristem actively produces new tissues at this time but goes into a resting 

 condition during the remainder of the year. 



As a result of the indeterminate scheme of growth the structure of the 

 tip of a branch or root is not the same as that of an older part of the same 

 branch or root. The growing point is actually in an embryonic state, and 

 the degree of maturity of structure increases proportionately with the 

 distance from that point. This situation makes it advisable to give a 

 unified treatment of the development, structure, and functioning of the 

 plant. 



GENERAL ORGANIZATION OF THE PLANT 



The seed and the embryo. All the highest plants produce special 

 reproductive bodies called seeds and form a group which derives its 

 scientific name from this characteristic — Spermatophyta (Greek, sperma, 



