38 JOUKNAL OF THE EOYAL HOKTICULTUEAL SOCIETY. 



Illustrations of Methods of Parasitism. — Commencing with the 

 mistletoe {Viscum album) of the family Loranthacece, containing thirteen 

 genera and 500 species, this has very glutinous berries, easily adhering to 

 boughs ; so that the embryo, by its radicle turning away from the light, 

 penetrates the bark of the tree, and then spreading along the cambium 

 layer thrusts " sinkers " inwards into the wood and roots into the cortex, 

 as well as sends shoots outwards into the air. The sinkers absorb water, 

 &c. Having green leaves they can utilise carbon-dioxide from the air, 

 and only require water and minerals from the host. They somewhat 

 resemble a scion on a stock. Loranthus europceus has been shown to 

 surrender organised matters to its host. It thus to some extent lives in 

 symbiosis with it. 



The genus Viscum has at least thirty species, while the British one 

 has several varieties. Such can only arise in "consequence of variations 

 in the trees and climate. In South Africa there are several species ; one 

 has very minute leaves, a feature in common with many herbs in that 

 excessively dry climate ; One in Australia is densely pubescent, from a 

 similar cause. 



Several members of the family are non-parasitic, as shrubs and trees, 

 showing that the parasitic habit is an acquired one. 



As terrestrial parasites I will take the leafy Cow-wheat, and a leafless 

 one, the Dodder. M. Leclerc du Sablon * has well described the forma- 

 tion of the suckers of the former, Melampyrum pratense, observing that 

 this plant, which possesses green leaves, is not always attached to host 

 plants ; for many individuals fix themselves, often indeed the greater 

 number, on decomposing pieces of wood or even simply to a packet of 

 humus very rich in organic matters. Melampyrum sometimes, 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 roots, by saprophytic and by parasitic suckers. The external 

 cause of their formation appears to be the stimulus excited by the con- 

 tact of a body, living or dead, which encloses nutritious (nitrogenous ?) 

 matters useful to the plant. The first stage of the development of a 

 sucker in Melampyrum consists of a slight protuberance, due to a 

 swelling of the cortex or superficial tissues of the root of the parasite. 

 The cell of the epidermis, where contact takes place, enlarges tan- 

 gentially 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 papillae or root-hairs. 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. If 

 such a sucker pass into dead vegetable matter, the extremities branch and 

 ramify through it. The corresponding cells, beneath this terminal cluster 

 of root-hairs, of one or two subjacent layers, undergo analogous modifica- 

 tions. 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 running 

 up the sucker into the root of the parasite and down to the root of the 

 host. These acquire spiral thickenings, and so become "tracheids" in 



* Bulletin de la Soc. Bot. de Fr. 1886, p. 154 ; and Ann. des Sci. Nat. ser. 7, 

 torn. 6, 1887, p. 90. 



