576 PARASITISM OF NOTOTHIXOS IXCANUS VAH. SI'BAUREUS, 



By the time that tlie parasitic haustorium has penetrated the bark of the host 



the iDhimule is withdrawn from the seed-coat which has shrivelled up. The 



haustorium subsequently increases in diameter rather than in length, and a con- 

 siderable swelling develops in the infected zone. 



Old Haustorium. 



The fully-developed haustorium, by solution and pressure, becomes closely 

 applied, at the apex and along the margin, to the host tissues. On examining a 

 longitudinal section of the haustorium, it is apparent that it has the form of a 

 massive wedge of tissue, pointed at the apex and very broad at the base. At 

 every part of its surface, it is in the closest contact with host-tissues — witii 

 xylem, phloem, and cortical elements. The cells at the extreme apex of the 

 haustorium are elongated, slightly papillate, with densely protoplasmic contents 

 and large nucleus (Text-fig. 7). 



During its gTowth the haustorium destroys, by solution and pressure, a 

 wedge-shaped mass of host-tissue until the apical region becomes applied to many 

 vessels of the host-stem. During its progTess through the peripheral tissues, it 

 must avail itself of the dissolved cell-contents and cell-walls of the host, as there 

 IS very little trace left of the cells which must have lain across the path of the 

 haustorium. As the growth of the haustorium is exceedingly slow, there is ample 

 opportunity for the opposing cells to be absorbed. The peripheral secretory 

 layer of the haustorium apparently exercises a chemical action upon the host- 

 cells. Occasionally an haustorial cell pushes the wall of a vessel inwards. Such 

 a result could only be attained in lignified walls by a previous softening action 

 developed by a secretion from the haustorial cell. Text-fig-ure 6 shows such a 

 development. 



In the centre of the haustorium there is a central mass of short reticulated 

 tracheids with cambiform cells occurring amongst them and always on each side 

 of them. Near the apex, isolated strands of these tracheids pass out towards the 

 margin, and become applied to vessels of the host, either to the tangential or 

 radial walls. This aiDplication of the parasitic tracheids to the water vessels of 

 the host is most exact, and is brought about by the differentiation of epidermal 

 and cortical cells of the haustorium into tracheids, as soon as the haustorium has 

 penetrated the xylem. The actual penetrating tip of the haustorium is singularly 

 free of tracheids. Text-fig. 8 shows a portion of a radial longitudinal section of 

 a host-stem through the haustorium, and the close application of the cells of the 

 haustorium to the host-vessels. Note the pits on the latter. Osmotic suction is 

 probably the means of absorption in this case. In Text-fig. 9, I have shown the 

 application of a tracheid of the haustorium to a vessel of the host. 



On either side of the central group of reticulate tracheids and cambiform 

 cells, there is a broad zone of cortical cells which, later, may become traeheidal 

 in structure and in function. Other cortical cells, on the contrary, may form 

 a connected strand of tracheids; these have a somewhat radial arrangement link- 

 ing up with the central strand. No sieve-tubes are present in the haustorium, 

 and Notothixos, therefore, recalls Viscum album and species of Loranthus in this 

 respect. 



If plastic food-stuffs are absorbed by the parasite from the host, apart from 

 those provided by the disorganisation of the host-tissues due to the development 

 of the haustorium, then no special provision is made for their conduction from 

 the haustorium to the parasite's main vascular bundles. I have searched care- 

 fully, but unsuccessfully, for sieve-plates between the sieve-tubes of the host and 



