DIV. I 



MORPHOLOGY 



191 



b 



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



stem of a Nettle or a young Willow shoot (Fig. 221 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 PRE- 



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



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. 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 tbe 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. 221 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 seeds of Orobanche (Broom rape), another 

 parasite, only germinate when in contact with the 

 roots of the host plant ; only its haustoria penetrate 

 the roots, and its light yellow, reddish-brown, or 

 amethyst - coloured flower -shoot appears above the 

 surface of the ground. Orobanche (Fig. 764), like Cus- 

 cuta, contains a small amount of chlorophyll. Both 

 are dreaded pests ; they inflict serious damage upon 

 cultivated plants, and are difficult to exterminate. 



A similar appearance to Orobanche is presented by 

 some plants which grow in the humus soil of woods, 

 and are, therefore, not at first sight regarded as para- 

 sites: certain Orchids (Neottia, Coralliorrhiza, Epipogori] and Monotropa. The 

 absence of chlorophyll, the reduction of the leaves to scales, and (in Coralliorrhiza] 

 the absence of roots also (cf. Fig. 222), are indications that these plants obtain 

 organic material from without. They cannot themselves directly utilise the humus, 

 but fungi, which obtain food from this, are harboured in their subterranean parts 

 as a MYCORRHIZA. A proportion of the fungal hyphae is later digested by the 

 plant. These cormophytes are thus in a sense parasitic on the fungi of the humus. 



In contrast to these parasites, which have come to be almost entirely dependent 

 on other plants for their nourishment, there are others which, to judge by 

 external appearance, seem to have a high degree of independence, since they 

 possess large green leaves and are capable of assimilation. They are, however, 

 parasitic, since they can only develop normally, when their roots are connected 

 by means of haustoria with the roots of other plants ; they are spoken of as 

 PARTIAL PARASITES. Thesium belonging to the Santalaceae, and the following 

 genera of the Rhinanthaceae, PJtinanthus, Euphrasia, Pedicularis, Bartsia, Alelam- 

 pyrum and Tozzia, may be mentioned as examples ; in Tozzia the parasitism is 

 especially well marked in the earliest developmental stages. 



FIG. 222. Rhizome of Corallor- 

 rhiza innata. a, Floral shoot ; 

 6, rudiments of new rhizome 

 branches. (Nat. size. After 

 SCHACHT.) 



