EVOLUTION OF PLANTS, AND DIRECTIVITY OF LIFE. 339 



plant food ; and they act as supports to the aerial part of the plant. 

 The forms of roots, more or less characteristic of species and varieties, 

 are not very numerous ; the commonest is long and cylindrical, such 

 as, e.g., the wild form of the carrot, turnip, radish, and parsnip, or 

 generally the tap or primary root of a dicotyledonous embryo. Under 

 cultivation in a wet and rich soil, it has varied to a conical form in 

 the carrot and parsnip ; into a spindle-shaped and globular one in the 

 radish and beet and in the rape and turnip respectively.* These 

 forms raised under cultivation are hereditary in ordinary garden soils ; 

 but they at once revert to the wild form if they be grown in a fine, 

 sandy soil. The globular type was produced by sowing the seed in 

 a stiff soil, which presumably prevents the downward penetration of 

 the tap-root. 



The cause of the change of form is the new conditions of life 

 provided in cultivation. The plant is stimulated by. much nourish- 

 ment to make larger leaves ; these then do more work, making more 

 starch and sugar than is required for annual growth ; hence it has to 

 be stored up, so the root enlarges to receive it. This " vegetative " 

 function being prolonged delays the process of reproduction by flowers 

 and seeds until it is too late in the season. The plant thus becomes 

 a biennial ; being provided with plenty of water, the leaves become 

 nearly or quite hairless. These garden forms are obviously thus the 

 result of response to the changed conditions of life. 



Peculiar forms with changed internal cellular structures are found 

 in many herbs and trees growing in marshy districts. Some trees, 

 as the deciduous cypress (Taxodium distichum), not infrequently 

 cultivated, have roots more or less hollow, which rise above the swamp, 

 erect, or as " knees," whereby the roots get aerated. One of these 

 trees near the Thames, in the gardens of Sion House, has numerous 

 " pneumatophores " (air-carriers) ; but a fine tree in the garden of the 

 Grand Hotel, Lyndhurst, in the dry sandy soil of the New Forest, 

 has none at all. 



Our little marsh-samphire (Salicornia herbacea), common in salt- 

 marshes, has its roots clothed with a spongy coating which provides 

 the plant with air in a similar way. 



On the other hand, some plants f growing in the deserts have knotty 

 swellings on the roots which store up water against the dry season. 



Clasping roots have been acquired to enable many plants of various 

 families to climb up walls, tree-trunks, &c, as our own ivy, members 

 of the Aroid family in tropical rain forests, epiphytic orchids, and many 

 others, showing the general property of adapting the roots to other 

 purposes than that of providing water and salts as in all ordinary plants. 



Tap-roots are always arrested in aquatic dicotyledons and all 

 monocotyledons, which have undoubtedly descended from the former. 

 Consequently their roots must arise from the base of the stem in 

 ascending series from the nodes ; the stem, increasing in size, becomes 



* The rape and turnip are varieties of the same species, Brassica campestris. 

 t E.g. species of Erodium, or Stork's-bill, in the desert near Cairo. 



