14 THE FLOWERING PLANT. 



and amount of branching ; (3) the thickness. The branching is 

 always monopodia!, that is to say, the branches are not formed 

 by forking, but arise as outgrowths from the side of a pre-existing 

 axis. The thickness partly depends upon the duration of life. In 

 annuals, which live for one year only, the parts of the root system 

 are usually of no great thickness. Biennials, which live for two 

 years, develop in many cases a greatly dilated tap-root, which 

 contains a store of reserve material that is used up the second 

 year, when flowering takes place, as in the carrot, turnip, and 

 radish. Perennials, which live for more than two years, fre- 

 quently have thickened roots, e.g., dahlia, where there is a bunch 

 of secondary roots swollen into spindle-shaped bodies. 



The structure of roots is somewhat complex, and can only be 

 very briefly dealt with here. Just as (see p. 8) a leaf is divisible 

 into three systems of tissue, so also is a root. This may be con- 

 veniently illustrated by the main root of a young bean-seedling 

 grown in damp sawdust. The younger part of this, i.e. the part 

 near the tip, will be covered by a thin ill-defined skin, the 

 epidermis, composed of a single layer of flattened cells, from 

 which numerous delicate unicellular root-hairs grow out (cf. iig. 

 7, H). Such hairs are seen much better in the case of mustard- 

 seedlings (fig. 3). If the root is cleanly cut across and examined 

 with a lens, an outer spongy-looking portion can be distinguished 

 from an inner denser portion. These correspond, respectively, 

 to a sort of external jacket, the cortex, and an internal firm 

 vascular cylinder (cf. fig. 2). In roots which, like those of the 

 bean, increase in thickness, the epidermis and cortex are early 

 thrown off, being replaced by a brown layer of cork formed in 

 the outer part of the vascular cylinder. This may easily be 

 made out in the roots of an old bean- plant. Suitable cross- 

 sections through the young root show that the secondary roots 

 run in to the vascular cylinder. If the cortex is peeled off, which 

 can be readily done, the secondary roots remain behind, attached 

 to the cylinder. These roots then arise endogenously, i.e., from 

 the internal tissue, and break their way through the cortex to 

 the exterior (figs. 5, B, and 7, H). This way of origin is charac- 

 teristic of all roots, primary, secondary, and adventitious. 



The cortex is composed of the second kind of tissue, which has 

 received the name of fundamental or ground tissue. This term 

 is a very broad and general one, and it must by no means be sup- 

 posed that all the component cells are of the same shape or nature. 

 On the contrary, several varieties of tissue may be included under 

 this system. The commonest and most important is parenchyma, 

 (fig. 7, H), which is made up of cells that are fairly equal in 

 length, breadth, and thickness. The cells in question are not 



