360 ECOLOGY 



vegetative organs are underground, its leaves are reduced 

 to scales, only the flowering shoot comes above ground, 

 and the plant is a sickly yellow or purplish colour with 

 little or no chlorophyll in its tissues. The rhizome is thick 

 and bears four rows of curious scale-leaves (Fig. 229, 1), 

 which are thick and fleshy, and back-rolled in such a way 

 as to give rise to a branched cavity opening to the exterior 

 by a narrow slit at the base. Very small animals often 

 enter the cavities and die there, and the products of their 

 decay may be absorbed ; while special cells in the walls 

 of the cavities may serve for the excretion of water. The 

 roots form disk-like attachments (Fig. 229, 2 s) on the roots 

 of trees such as Hazel, Elm, and Beech, and suckers from 

 the disks enter the tissues of the host and absorb nutri- 

 ment ; it is also probable that some nutriment is absorbed 

 from the humus after the manner of saprophytes. 



Several species of Broomrapes occur in Britain and are 

 parasitic on the roots of such plants as Broom, Gorse, 

 Clovers, Hemp, and Ivy. They are dirty white or yellowish 

 in colour, or tinged with pink and, like the Toothwort, 

 send only their flowering shoots above ground. 



An extreme example of parasitism is found in the Dodders. 

 These, however, are stem-parasites, and they possess many 

 remarkable features. The seed, on germination, sends its 

 radicle a very short distance into the ground, while the 

 slender stem nutates, and is sensitive to contact like 

 a tendril (Fig. 230). If it comes in contact with a ' host ', 

 it twines round the stem and sends a sucker or haus- 

 torium (Fig. 231) into it, and both the wood and the bast 

 of the haustorium form a union with the corresponding 

 tissues of the host. By this means the Dodder can obtain 

 the whole of the food it requires. The root now dies away, 

 and the slender stem, which is without chlorophyll and does 

 not bear any green leaves, subsists entirely at the expense 

 of the plant on which it preys. As new branches arise they 



