40 JOUENAL OF THE ROYAL HORTICULTURAL SOCIETY. 



cells of the aerial stem become "claspers," and then the sucker penetrates 

 through the middle of them. This fixation of the cup-like clasper appears 

 indispensable to complete the development of the penetrating sucker. 

 For when the epidermal cells do not prolong themselves into clasping 

 organs the sucker which arises within them undergoes an arrest of 

 development, and does not form vessels. From this it appears that either 

 the hairs are the vehicle of irritation, which must be kept up, and by 

 conduction influences the deeper tissues, which in response begin to form 

 the absorbing organ from within ; or it may be that the " purchase " 

 required for the necessary force to drive the sucker root into the host is 

 insufficient. 



The epidermal cells between any two suckers help mechanically to fix 

 the plant. Their cuticle presents fine indentations which mould themselves 

 upon the corresponding asperities, fitting into depressions of the cuticle 

 between the adjacent cells of the host. And they adhere still more firmly 

 by means of a gummy secretion. 



The means by which the root sucker or haustorium penetrates the 

 host is mainly by pressure, leverage being secured by the closeness of the 

 coils together with the superficial adhesion mentioned above. Besides 

 this the central epidermal papillate cells dissolve a passage and absorb 

 the nutriment which is conveyed to the root immediately below them. 

 As they are not the true sucker (as in the Euphrasiece), this epidermal 

 sucker is called the " pre-haustorium." 



Experiments show that although no chlorophyll is visible in Cuscuta 

 as a rule, it can be induced to form it ; for if short lengths be cut off 

 a branch, the failure of sufficient nutrition — in the supply of food from 

 below, necessary for the formation of haustoria, being removed — is now 

 compensated for by the immediate formation of chlorophyll. Similarly if 

 the Cuscuta be attached to a feeble host, or to one which cannot nourish 

 it properly, as Euphorbia, it puts on chlorophyll, so as to be more inde- 

 pendent of its host. When, however, fresh haustoria have been made on 

 a fitting host, then the chlorophyll disappears and perfect parasitism is 

 again recovered. 



Parasitism, Potentially General. — That the power to become parasitic 

 is a general one seems obvious from the fact that parasites occur in widely 

 different orders, and that even a passion flower happening to have its roots 

 adjoining those of a Euonymus japonica became parasitic upon it.* But 

 why some roots when in contact with others are stimulated by the latter 

 into producing suckers, rather than vice versa ; or why they are not 

 naturally parasitic, is as obscure as the answer to the question why some 

 marsh plants have become insectivorous, while others associated with 

 them have not. 



The point, however, which may be insisted upon is, that parasitism is 

 a consequence of organic irritation, as described in Melampyrum. This, 

 when once fully set up and continued for generations, induces a hereditary 

 predisposition to parasitism ; as seen in many of the tribe EuphrasiecB of 

 the order Scrophularinece. This, again, by further degradation in the 

 structure of the parasite, as in all which are devoid of chlorophyll, becomes 

 a fixed and absolutely necessary habit of life. 



* Rev. Gen. de Bot. vol. xvi. p. 453. 



