228 BOTANY PA in t 



exercised by the chlorophyll. With the diminution or complete 

 disappearance of the chlorophyll, and consequent adoption of a 

 dependent mode of life, the development of large leaf surfaces, so 

 especially fitted for the work of assimilation, is discontinued. The 

 leaves shrink to insignificant scales, for with the loss of their assimi- 

 latory activity the exposure of large surfaces to the light is no longer 

 essential for nutrition. For the same reason active transpiration 

 becomes unnecessary ; the xylem portion of the vascular bundle 

 remains weak, and secondary wood is feebly developed. In contrast to 

 these processes of reduction resulting from a cessation of assimilation, 

 there is the newly developed power in the case of parasites to penetrate 

 other living organisms and to deprive them of their assimilated products. 

 In saprophytic plants, however, where the question is merely one of 

 absorbing nourishment from organic remains, the external adaptations 

 for taking up nourishment continue more like those for absorbing 

 the mineral salts from the soil, for it then depends only upon an 

 intimate contact with the decaying substances. 



Cuscuta europaea (Fig. 202), a plant belonging to the family of the Convolvu- 

 laceae, may be cited as an example of a parasitic Phanerogam. Although, through 

 the possession of chlorophyll, it seems to some extent to resemble normally assimi- 

 lating plants, in reality the amount of chlorophyll present is so small that it is 

 evident that Cuscuta (Dodder) affords an example of a well-equipped parasite. 



The embryonic Cuscuta plantlet, coiled up in the seeds, pushes up from the 

 ground in the spring, but even then it makes no use of its cotyledons as a source 

 of nourishment ; they always remain in an undeveloped condition (Fig. 202 at the 

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

 which soon dies off. The seedling becomes at once drawn out into a long 

 thin filament, the free end of which moves in wide 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 farther at the expense of the nourishing matter drawn from the 

 other extremity of the filament, which then dies off (t) as the growing extremity 

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

 ultimately into contact with a suitable host plant, such as, for example, 

 the stem of a Nettle or a young Willow shoot (Fig. 202 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 plan.t, 

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

 HAUSTORIA are soon followed by special organs of absorption, the HAUSTOKIA (If). 

 These arise from the internal tissues of the parasite, and possess, in a marked 

 degree, the capability of penetrating 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 hypha-like 

 filaments produced from the main part of the haustoria penetrate the soft 

 parenchyma and absorb nourishment from the cells. 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 



