sect, n PHYSIOLOGY 209 



of nourishment ; they always remain in an undeveloped condition (Fig. 185, at the 

 right). Nor does any underground root system develop from the young rootlet, 

 which however soon dies off. The seedling hecomes at once drawn out into a 

 long thin filament, the free end of which moves in broad circles, and so inevitably 

 discovers any plant, available as a host, that may be growing within its reach. 

 In case its search for a host-plant is unsuccessful, the seedling is still able to creep 

 a short distance further at the expense of the nourishing matter drawn from the 

 other extremity of the filament, which then dies off (<) as the growing ex- 

 tremity lengthens. If the free end, in the course of its circular movements, comes 

 ultimately into contact with a. proper nourishing plant, such as, for example, 

 the stem of a Nettle or a young Willow shoot (Fig. 185, in the centre), it twines 

 closely about it like a climbing plant. Papillose protuberances of the epidermis 

 are developed on that side of the parasitic stem in contact with the host-plant, 

 and pierce the tissue of the host. If the conditions are favourable, these pre- 

 haustoria are soon followed by special organs of absorption, the haitstokia {H). 

 These are peculiarly developed adventitious roots which arise from the internal 

 tissues of the parasite, and possess, in a marked degree, the capability of penetrat- 

 ing to a considerable depth into the body of the host-plant by means of solvent 

 ferments and the pressure resulting from their own growth. They invade the tissues 

 of the host, apparently without difficulty, and fasten themselves closely upon its 

 vascular bundles, while single hyphal-like filaments produced from the main part of 

 the haustoria penetrate the soft parenchymatous cells and absorb nourishment from 

 them. . A direct connection is formed between the xylem and phloem portions of 

 the bundles of the host-plant and the conducting system of the parasite, for in the 

 thin-walled tissue of the haustoria there now develop both wood and sieve-tube 

 elements, which connect the corresponding elements of the host with those of the 

 parasitic stem (Fig. 185, at the left). Like an actual lateral organ of the host- 

 plant, the parasite draws its transpiration water from the xylem, and its plastic 

 nutrient matter from the phloem of its host. The haustoria of Orobanche (Broom 

 rape), another parasite, penetrate only the roots of the host-plant, and only its 

 light yellow or reddish-brown or amethyst-coloured flower-shoot appears above the 

 surface of the ground. Orobanche, like Cuscuta, also contains a small amount of 

 chlorophyll. Both are dreaded pests ; they inflict serious damage upon cultivated 

 plants, and are difficult to exterminate. 



Many parasitic plants, especially the Eafflesiaceae, have become so completely 

 transformed by their parasitic mode of life that they develop no apparent vegetative 

 body at all ; but grow altogether within their host-plant, whence they send out at 

 intervals their extraordinary flowers. In the case of PilostyUs, a parasite which 

 lives on some Asiatic species of Astragalus, the whole vegetative body is broken 

 up into single cell filaments, which penetrate the host-plant like the mycelium of 

 a fungus. The flowers alone become visible and protrude from the leaf-axils of 

 the host-plant. 



In addition to these parasites, which have come to be absolutely- 

 dependent upon other plants for their nourishment, there are certain 

 parasites which, to judge by external appearances, seem to be quite 

 independent, for they possess large green leaves with which they are 

 able to assimilate vigorously. In spite of this, however, these plants 

 only develop normally, when their root system is in connection with 

 the roots of other plants by means of disc-shaped haustoria. Thesium, 

 belonging to the Santalaceae, and the following genera of the Rhynan- 



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