MODIFICATIONS OF FORM 167 



bulb-grower secures normal perennation, with seasonal flowering; the 

 purchaser is apt toforget that its continued success depends on nutrition 

 being maintained till the green leaves shrivel, and functional activity 

 ceases for the year. This dormant state, in which the bulb or corm is 

 bought, is itself an accommodation to seasonal drought. The bulb- 

 habit is widely spread, but it is specially characteristic of countries like 

 Southern Europe and the Cape, with a moist spring, but a dry and 

 hot summer. 



Symmetry, and its Modifications. 



The Root-System, developing in the soil, finds a medium in which 

 the conditions of temperature and moisture are relatively constant ; 

 but its form is liable to be strongly influenced by the texture of the 

 soil. Growing roots yield readily to the mechanical resistance thus 

 offered by any large obstacle. But if the roots develop in water, or 

 if the texture of the soil be fine and uniform, as it is in prepared garden 

 soil, the root-system develops with a regular symmetn,'. When, as 

 in Dicotyledons, there is a definite tap-root, this grows vertically 

 downwards, and the lateral roots radiate from it equally in all direc- 

 tions. Except for the effects of mechanical resistance, the root- 

 system of ordinary plants shows little departure from this regular 

 symmetry, while the individual root is typically cylindrical. It is 

 dift'erent where, as in epiphytes, the roots are aerial. Thus those 

 Orchids, which normally grow perched on the branches of trees, but are 

 cultivated in hanging baskets or on cork, often have roots of a flattened 

 form, which follow closely the surface to which they become attached. 

 Occasionally they may even become green, and act as eff'ective organs 

 of Photo- Synthesis. But these are exceptional cases. Speaking 

 generally the root of Flowering Plants retains its uniform cylindrical 

 outline, and the whole root-system is built up as regularly or radially 

 symmetrical. This fact may rightly be related to the uniformit}' of its 

 usual surroundings. 



The Shoot, on the other hand, is exposed to much more varied con- 

 ditions than the root. It may be developed in water, still or moving ; 

 or if developed in air, it may be subjected to various degrees of lighting 

 and moisture, and to winds from any quarter, as well as to the various 

 incidence of gravity. It is possible to trace, in the diff'erent forms of 

 the shoot which we see, a relation to and fitness for its surroundings. 

 It would be strange if the shoot, which is so adaptable individuaUy 

 as we have seen it to be (Chapter VIII.), should not show variety 

 of conformation in the race, seeing that its surroundings are so 



