226 



BOTANY OF THE LIVING PLANT 



Fig. 153. 



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 Hazel 

 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 



thfir 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. 153). 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, which provide no sem- 

 blance of stem, leaf, or root. The vegetative system is, in fact, reduced to the 



Fig. 154. 

 Flower-buds of Rafflesia 

 bursting their way out 

 from the root of Cissus. 

 (After Robert Brown.) 

 Much reduced. 



