IRREGULAR NUTRITION 



193 



FIG. 144. 



Median section of a young plant of 

 Orobanche seated upon the root of its 

 host. (After Hovelacque.) ( x 20.) 



and cortex, thus tapping both storage and conducting tissues. Where 

 the sucker impinges upon a vascular strand a continuous xylem- 

 connection may be established ; in the 

 phloem also a close relation of the sieve- 

 tubes of the parasite with those of the 

 host has been shown. 



The Broomrape (Orobanche), and the 

 Toothwort (Lathraea) are further examples 

 of parasites with complete physiologi- 

 cal dependence. Both of these are root- 

 parasites, with attachment to the host by 

 haustoria, which penetrate the tissues. The 

 Toothwort which infests the roots of Haze! 

 is classified in the Scrophulariaceae, close to 

 the Eyebright and Yellow Rattle, which are 

 themselves partial root-parasites. But it 

 differs from them in having become entirely dependent physiologically upon 

 its host. The leaves are still represented on the underground shoot, and 

 their curiously reduced and altered form gives rise to 

 the name of Tooth- Wort. But the flowering shoot 

 rises above ground, displaying flowers with structure 

 characteristic of the Family. 



The Broom- rapes (Orobanche), which attack various 

 plants, woody or herbaceous, are closely related to 

 Lathraea. They show a greater modification of the 

 shoot, which attaches itself on germination to the 

 root of the host, developing' a brown tuberous body, 

 without leaves, and shut off from the light. By means 

 of a sucker it burrows with a broad surface into the root 

 of the host plant, and establishes a close relation with 

 its conducting tissues (Fig. 144). The flowering shoot 

 with its brownish leaves rises above ground, bearing 

 numerous flowers. Their structure shows that it is a 

 form related to the Toothwort, but its vegetative 

 system is still more reduced, leaves being absent from 

 the base of the tuber. This reduction runs parallel 

 to but distinct from that seen in Convolvulus and 

 Dodder. The two sequences provide a good example 

 of homoplasy, or parallel development, and show 

 that parasitism may originate separately in distinct 

 families, though the steps of the consequent modifi- 

 cation may be alike. 



An example of a still further reduction of the vege- 

 tative system of a complete parasite is seen in 

 Rafflesia, which grows enclosed within the tissues of 

 its host. It infests the stems and roots of Cissus, 

 traversing the tissues with branched filaments of cells. At first there 

 is nothing that can be recognised as stem, leaf, or root. The vegeta- 



B.B. N 



FIG. 145. 



Flower-buds of Rafflesia 

 bursting their way out from 

 the root of Cissus. (After 

 Robert Brown.) Much re- 

 duced. 



