RELATION OF STRUCTURE TO FUNCTION IN ROOTS. 763 



here, still, by the development of poisonous and disagreeable substances, the sub- 

 terranean, burrowing insects are, as far as possible, kept away. It is well 

 known that roots are particularly rich in poisonous alkaloids, in resins which 

 are repulsive to animals, in bitter substances and the like; these parts of plants 

 are well known as providing many drugs of the pharmacopeia. These do not 

 indeed afford an infallible protection against all attacks from animals, but that 

 a partial safeguard at least is obtained by the storing up of certain materials 

 seems very probable by the following observations. The field-mice in a garden 

 at Innsbruck once caused great havoc under the winter coat of snow, and various 

 roots were gnawed by them; but the roots and root-stock of the Soap wort 

 (Saponaria officinalis), containing quantities of poisonous saponin, were always 

 left untouched by them. The bitter roots of gentians (Gentiana punctata, lutea, 

 Pannonica), which are very rich in reserve-foods, and which grow in deep alpine 

 meadows riddled by mice, were never seen to be attacked by a single animal. 

 This was also the case with the thick tap-roots of the poisonous monkshood, the 

 massive roots of rhubarb-plants and of many Umbelliferae, which are all abundantly 

 supplied with starch and other food-stuffs, and therefore would afford an excellent 

 food for herbivorous animals under stress of hunger. 



When the parenchymatous tissue surrounding the central strand of the con- 

 ducting bundles in subterranean roots serves not only as an agent for protecting 

 against lateral pressure, but also for the storing up of food-materials, and in 

 addition possesses contrivances for warding off voracious animals, the structure of 

 the roots is much more complicated than in cases where it affords protection against 

 lateral pressure alone. There are also very many different developments of par- 

 enchymatous tissue on the periphery of subterranean roots in accordance with the 

 various demands necessitated by the conditions of the habitat and the peculiar 

 mode of life of the species. In aquatic roots the need for abundant ventilation has 

 also to be considered, and the storage of reserve-foods in these organs must be 

 avoided since the increase in weight, due to the massing of reserve-food, might 

 draw the floating water-plant down into the water at an unsuitable time. 



A storage of food-materials in the special tissue developed at the growing root- 

 tip, and known as the root-cap, would also be unsuitable. In subterranean roots 

 the root-cap only protects the delicate dividing and multiplying cells at the growing 

 end. The pressure to which these continually dividing cells are exposed in their 

 penetration into the ground is much greater than that operating on the fully 

 formed parts behind the root-tip. The growing point of the root has to push on 

 one side hard grains of sand and other particles of earth, and to make a hole like a 

 ground-auger in which later on the fully developed root can take up its position. 

 The root-cap may be compared to a shield which is formed by the growing and 

 therefore advancing cells in the direction required, these constantly pushing it in 

 front of them. This shield is always being supplemented and renewed by the 

 growing tissue. The half of the root-cap adjoining the growing tissue consists of 

 angular, closely-fitting cells; the outer half, directed towards the soil, consist* of 



