760 RELATION OF STRUCTURE TO FUNCTION IN ROOTS. 



reserves of food necessary for the founding of a new establishment. It would be 

 of no use, and contrary to the economy of plants, if reserve materials were de- 

 posited in any other parts, say in the stem or roots, since these parts shrivel and 

 dry up as soon as the seeds are dispersed, and the energy expended in the manu- 

 facture and storage of starch, fat, sugar, and other reserve food would be expended 

 in vain. The roots of annual plants are therefore satisfied with delivering the 

 necessary water and the required amount of food-salts to the plant during its short 

 period of vegetation, and with providing a suitable attachment to the substratum; 

 they waste no energy in founding subterranean reservoirs. In biennial and 

 perennial plants it is quite otherwise. Biennial plants — as well-known examples 

 of which may be taken the various roots used as vegetables, the Carrot {Daucus 

 Carota), the Turnip (Brassica Rapa rapacea), and the Beet-root (Beta vulgaris 

 rapacea) — develop during the first year a very short stem with foliage-leaves 

 crowded in a rosette, and a thick, fleshy tap-root (radix palaris), or turnip-shaped 

 root (radix napiformis). When vegetative activity recommences in the second 

 year, an erect shoot with foliage and flowers is constructed at the expense or at 

 any rate with the help of the materials stored up in the thickened root; fruits are 

 produced from the flowers, and after the ripening of the seeds the whole shoot dies 

 oflT together with the exhausted roots. In perennial plants the roots, when they 

 serve for the reception of abundant reserve-materials, are usually considerably 

 thickened; but in these plants it is the clustered root-fibres springing from the 

 lower end of the underground part of the stem, after the primary root has died 

 off", which undergo this development. When the thickening is symmetrical and 

 fusiform, as in the Orpine (Sedum Telephium) and in the white-flowered Orobus 

 Pannonicus, the roots are called fusiform (radices grumosce); when they are 

 swollen at intervals into knots, as in the Dropwort (Spiraea Filipendula), and in 

 the yellow Day-lily (Hemerocallis jiava), they are termed nodose (radices nodosae). 

 Many of our terrestrial orchids have two kinds of roots united in a fascicle, long 

 cylindrical vermiform roots and short thick roots filled with reserve-materials 

 which look very like tubers, and are called tuberous roots (radices tuberosce). The 

 Mediterranean flora and that of steppes, where in midsummer the vital activity of 

 plants is much reduced, are particularly rich in plants whose roots are developed 

 as storehouses for reserve materials. Plants of widely diflTerent families (e.g. 

 Ranunculus Neapolitanus, Centaurea napuligera, Valeriana tuberosa, Rumex 

 tuberosus, Asphodelus albus) there form thickened fascicled roots crowded with 

 reserve-materials which pass through the dry season unharmed underground, and 

 in the next period of vegetation supply the materials for the rapid construction 

 of epigeal foliage and flowering shoots. These thickened bundles of roots are 

 characteristic of the perennial, parasitic species of the genus Pedicularis. They 

 serve for the storage of reserve foods, for the fixing of the plant, and for the absorp- 

 tion of nourishment, but the latter function is here carried on by means of suckers, 

 which are developed at the end of the thickened fusiform fibres, and which attach 

 themselves to the roots of the host plants in the manner described on p. 179. 



