PARASITIC AND SAPROPHYTIC PLANTS. 



39 



communication with the spiral vessels in the root of the parasite. At the 

 other end the bundle of root-hairs, now modified as absorbing organs, 

 penetrates the cortex of the root of the host plant, dissolving and con- 

 suming it by means of a ferment ; but they are stopped by the woody 

 central cylinder, which resists the solvent action of the ferment ; con- 

 sequently they bend round and invest this central cylinder. 



The part of the sucker of Melampyrum which enters the host plant 

 has thus the epidermal layer for its origin. In BhinantJius, the Yellow 

 Eattle, additional root-hairs {i.e. besides those forming the penetrating 

 cone) can invest the root of the host and take part in absorbing nourish- 

 ment from it. These, however, do not become spiral tracheids like the 

 former. 



Other genera show various slight differences, but the general principle 

 on which the suckers are formed in the EuphmsiecB is the same ; that is 

 to say, the sucker is epidermal. From the above brief description it 

 would seem that these leafy parasites supply us with a somewhat primitive 

 type of parasitism ; first, in their having green leaves, and secondly, in 

 that the effect of the irritation is more superficial than in the greatly 

 degraded state of leafless and greenless parasites ; such as Cusciita, to be 

 next described, in which the irritation acts more minutely on the deeper- 

 seated cortical tissues, wherein is formed the absorbing organ ; while in 

 the Euphrasies, as we have seen, the suckers are formed by the epidermis 

 alone. 



The method by which an aerial leafless parasite fixes itself to a 

 host plant is well illustrated by Cuscuta, the Dodder. The seed of this 

 plant has no cotyledons and resembles a minute coiled thread. It 

 germinates by fixing its club-shaped root-end in the soil and draws up 

 water by means of root-hairs ; it then sends up a thread-like stem, which 

 circumnutates like a tendril in search of a support. As soon as it comes 

 in contact with a suitable host plant it quickly forms an adhesive 

 " haustorium," and from that moment the lower part dies and the 

 dodder becomes entirely parasitic and grows rapidly all over the host 

 plant. 



If the seedling fails to find a host its stem falls to the ground, and 

 provided the soil be moist, the apex will grow while the root-end dies. 

 The nourishment is then continually being transferred from one end to 

 the other as the stem, as well as the root-end, dies from below upwards. 

 The little plant thus ''moves " along, as it were, by growth until it may 

 come in contact with some suitable plant. If it fail to do so it will of 

 course perish in time. 



In circumnutating it forms two kinds of spirals. If it be growing 

 rapidly, the coils are loose and it fails to form haustoria ; but when this 

 period of rapid growth is over, it coils in more horizontal spirals closely 

 adpressed to the host plant. It is on these alone that the haustoria are 

 formed. 



The sucker originates at a point ivithin the cortex, where a focus of 

 "merismatic" or rapidly dividing embryonic tissue is formed. This 

 invades the central region of the stem of the dodder and also extends 

 outwards. Vessels are now formed in this, and become united to the 

 central cylinders of both host and parasite. In Cuscuta the epidermal 



