BIV. I 



MORPHOLOGY 141 



developed from the stem. All increase of the root-system, however, 

 depends on a supply of organic food materials manufactured in the 

 leaves. Thus the further development of the crown of foliage and of 

 the root-system are intimately related to one another. The increase 

 in size of the shoot- and root-systems further presupposes that a 

 sufficient number of conducting tracts in the stems and roots can be 

 developed, both for water and for organic materials, and that the stem 

 should be strong enough to support the increasing weight even when 

 exposed to wind. There is thus an intimate connection between the 

 size of the cormus and the formation of conducting tracts in its axes 

 and the rigidity of the shoot. 



The rigidity requires to be greater the larger the plant becomes 

 and the longer it lives. Plants or shoot-systems which only live for a 

 limited period and die off after bearing reproductive organs have 

 usually herbaceous structure (HERBS). Large cormi which live for 

 many years and bear fruit repeatedly have as a rule the rigidity 

 of their stems and roots increased by the formation of wood. Such 

 woody plants are called SHRUBS if they do not exceed a moderate 

 height, and retain their lateral shoots so that their branches are 

 formed near the ground. They are called TREES ( 75 ), on the other 

 hand, if they attain a greater height, have a main stem or trunk 

 (which must have the type of rigidity possessed by a pillar), and 

 usually lose their lower branches at an early period. 



In catalogues and descriptions of plants the duration of the period of growth 

 is usually expressed by special symbols : thus indicates an annual ; a biennial, 

 and ^ a perennial herb ; \i is employed to designate shrubs, and for trees the 

 sign ^ is in use. A special type of tree is found in the columnar and usually 

 unbranched stems of Palms and Tree-ferns ; in them secondary thickening, and 

 a true woo ly mass resulting from this, are wanting. 



The requirements, both as regards the number of conducting 

 tracts and the necessary rigidity, are met in a variety of ways in 

 cormophytic plants. In the first place, there are plants in which the 

 main axis of the seedling and any lateral branches that arise attain 

 a sufficient thickness and develop sufficient mechanical and conduct- 

 ing tissues before growth in length ; when this takes place later the 

 thickness is adequate for the future increase in size of the plant. 

 The primary root in such cases remains thin and usually dies off 

 early, while as many roots as are necessary arise from the basal 

 portion of the shoot. Secondly, there are plants in which long 

 slender stems and roots with only a few conducting and mechanical 

 elements are first developed. A limit would soon be set to the supply 

 of water to the leaves and of nutritive material to the root-system, 

 and thus to the increase in size of the plant, by the small number of 

 conducting elements in the primary stem and root. Provision is, 

 however, made for an increase in the conducting and mechanical 



