140 BOTANY PART i 



Many Dicotyledons (e.g. Lupin, Oak) and Gymnosperms (Pine) 

 possess a radial MAIN-ROOT or TAP-ROOT (Fig. 158) which, from the 

 seedling onwards, forms the downward continuation of the main stem 

 and grows vertically down into the soil (orthotropous). On this 

 radial lateral roots of the 1st order arise, which penetrate the soil 

 horizontally or obliquely (plagiotropous). The lateral roots of the 

 2nd order arise in turn on those of the 1st order. They tend to 

 grow on all sides from the latter so that the branches of the root- 

 system penetrate the soil as uniformly as possible in all directions, 

 and, as branching continues, do not leave a cubic centimetre unused. 



A tap-root is usually wanting in Monocotyledons since it becomes 

 arrested in the seedling stage. In its place numerous roots arise 

 from the base of the stem and penetrate the soil vertically, obliquely, 

 or horizontally. They branch monopodially, bearing lateral roots of 

 successively higher orders which penetrate the soil in all directions. 

 In the Wheat, for example, there is no tap-root, but the root-system 

 continues to extend in a horizontal plane. 



The length of all the roots of a plant taken together is surprising. 

 Thus for a plant of Wheat it may amount to some hundreds of metres. 



Some of the roots of trees in tropical forests are developed in a 

 peculiar fashion. The extraordinarily high and thick stems of many 

 such trees are supported at the base by strong vertically - placed 

 BUTTRESS-ROOTS. In other cases support is given by aerial roots 

 growing down from the branches to the earth and attaining the 

 thickness of woody trunks (PROP-ROOTS, e.g., in species of Ficus). 



(e) Secondary Growth in Thickness of the Cormus 



It has been seen that the additions to the root and shoot 

 made by the increase in number of the meristematic cells in the 

 growing points increase in length as they mature. A certain increase 

 in thickness of the parts is associated with this growth in length 

 this depends on the enlargement of the cells on passing from 

 the meristematic condition and not on increase in their number 

 (PRIMARY GROWTH IN THICKNESS, cf. Figs. 98, 100, 102, 115). This, 

 as a matter of fact, is slight, but is often followed in stems and roots 

 by processes of growth that will now be considered. 



The larger the shoot- system becomes the more readily will it 

 escape overshadowing by other plants and form more organic 

 material. Thus in many plants the growth of the small seedling 

 with a few leaves leads, with the accompanying branching, to a cormus 

 of the size of a large tree bearing a very large number of leaves. 

 The increase in the aerial shoot-system and in the number of leaves 

 makes progressively great demands on the water supply from the 

 roots, which can only be met by the increase of surface and the 

 branching of the root-system ; in many cases additional roots are 



