62 PARASITES AND SAPROPHYTES [CHAP. 



rich in organic matters. The cow- wheat some- 

 times, indeed, has no attachment to a host -plant 

 at all, and so far becomes a saprophyte instead. 

 It has, therefore, three methods of obtaining 

 nourishment, by normal, by saprophytic roots, 

 and by parasitic suckers. The external cause 

 of their formation appears to be the stimulus 

 excited by the contact of a body living or dead, 

 which encloses nutritious matters useful to the 

 plant. The first stage of the development of 

 a sucker in this plant consists of a slight pro- 

 tuberance due to a swelling of the cortex or 

 superficial tissues of the root of the parasite (a). 

 The cell of the epidermis, where contact takes 

 place, enlarges tangentially until it acquires a 

 breadth six to eight times greater than its 

 primitive size. It is thus enabled to give rise 

 to a cluster of cells which elongate outwards 

 into papillas or root-hairs (a). These latter now 

 undergo a remarkable change. They form a 

 conical bundle, more or less compact, and 

 are thus enabled to penetrate into the tissues 

 of the host-plant (c). If such a sucker pass into 

 dead vegetable matter, the extremities branch and 

 ramify through it (6). The corresponding cells, 

 beneath this terminal cluster of root- hairs, of 

 one or two subjacent layers, undergo analogous 

 modifications. Each of them is now transformed 

 into a row of cells along the axis of the sucker 

 till there are about ten rows forming a bundle 



