SECT. II PHYSIOLOGY 223 



substances are, that the parasite absorbs and requires for successful 

 growth, is, however, difficult to determine. Since frequently only 

 organisms of a definite natural group (family, genus, species) are 

 attacked by one species of parasite it may be assumed that the latter 

 makes quite specific demands as to the quality or ciuantity of its 

 nutriment. This assumption is supported by the fact that we are 

 unable to cultivate most parasites apart from their hosts. 



In phanerogamic parasites especially the changes in structure of the organs, 

 which stand in relation to the modification in their function due to parasitism, are 

 especially evident. 



From the clianges in their external appearance it is evident how far-reaching 

 is the influence exercised by the chlorophyll. With the diminution or complete 

 disappearance of the chlorophyll, and consequent adoption of a dependent mode o 

 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 assimilatory 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 develojjed 

 power in the case of parasites to penetrate other living organisms and to deprive 

 them of their assimilated products. In saprophj'tic plants, however, where the 

 question is merely one of absorbing nourishment from organic remains, the 

 external adaptations for taking up nourishment continue to be 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 europea (^■') (Fig. 196), a plant belonging to the family of the Convolvu- 

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

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

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

 evident that Cuscuta (Dodder) affords an example of a very complete 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 means 

 of nourishment ; they always remain in an undeveloped condition (Fig. 196 at the 

 right). Xor does any underground root-system develop from the young rootlet, 

 which soon dies oil. 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 tlie 

 other extremity of the filament, which then dies ott" {t) as the growing extremity 

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

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

 stem of a Nettle or a young "Willow shoot (Fig. 196 in the centre), it twines 

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

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

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

 HAUSTORIA are soon followed by special organs of absorption, the hai'storia {H) 

 (of. p. 49). These arise from the internal tissues of the parasite, and possess, in a 

 marked degree, the capability of pjnetrating to a considerable depth into tlie body 



