HAUSTORIAL ORGANS OF PARASITES. 



257 



mycelium of a Fungus. Occasionally the strands may expand to form 

 single-layered plates of cells, or even larger cell-masses ; the largest of 

 these expansions are the so-called " floral cushions " which ultimately 

 give rise to the flowers. 



Rafflesia is parasitic on the stems and roots of spp. of Cissies. Accord- 

 ing to Schaar, the thallus of R. Rochussmii consists mainly of hypha- 

 like strands of cells which traverse the protein-conducting secondary 

 leptome of the host (Fig. 103). Other branches pass radially through 

 the cambial zone and the secondary xylem, thus simulating medullary 



Fig. 103. 



Filaments of the " thallus " of Rafflesia Rochussenii in the secondary leptome of a 

 root of Cissus sp. A. Hadial L.8. through the root. B. Tangential L.S. (The leptome 

 consists mainly of sieve-tubes and companion-cells.) 



rays. Others, again, run in the genuine secondary xylem- and phloem- 

 rays, which contain starch. These radial thallus-strands give off 

 numerous longitudinally directed branches especially in the region of 

 the secondary phloem, which follow a very sinuous course. It is note- 

 worthy that any radial thallus-strands that may have travelled through 

 the phloem as far as the outermost sieve-tubes which are entirely 

 devoid of contents, turn back and once more grow towards the centre. 

 Evidently the parasite by no means pervades the tissues of the host at 

 random, but, on the contrary, confines its attention to the tissues that 

 are richest in nutrient materials. In passing from phloem to xylem, 

 the thallus filaments have of course to traverse the cambial zone ; there 

 must, therefore, be some arrangement which prevents the secondary 

 growth of the root from, breaking the connection between the strands 

 in the phloem and those in the xylem. This continuity is, in fact, 

 ensured by the circumstance that the radial thallus strands themselves 

 become meristematic at the level of the cambium. In this region 



R 



