THE HAUSTORIUM. 9 



septum separating the woody cylinder of the root (of the parasite) from 

 that portion of the cushion in which differentiation has already begun to 

 take place. The axial cuboid cells of the cushion run into and form the 

 sinker portion of the haustorium, and as they do so they become the haus- 

 torium. The terminal cells of the sinker are much elongated and form 

 the cells actively concerned in the absorption of the host tissues. The cork 

 of the haustorium is not wholly confined to the part in contact with the 

 soil, but is to be found also on the haustorium so far as the inner face of 

 the ring of mechanical tissue in the cortex of the host. (See plate 3.) 



In form the mature haustorial cushion is about the same as the imma- 

 ture, except that it is somewhat flattened. None seen were much over 

 6 mm. in diameter; they are usually much smaller than this. 



The mature haustorial cushion has a peculiar structure, not easily 

 described. In longitudinal median section it has three easily distinguish- 

 able parts, of which the upper constitutes a zone, practically a septum, of 

 parenchyma, a middle zone in which there is so much tissue differentiation, 

 and a lower zone, mainly parenchyma in contact with the sinker. 



In transverse section the middle zone of the cushion is seen to be made 

 up of an outer or cortical part, which is thin- walled parenchyma only, and 

 an inner part which constitutes the main portion of the sinker in this sec- 

 tion. This is composed almost entirely of tracheid-like cells, which, as a 

 mass, bow o*-t like the staves of a barrel. The tracheid-like cells end at 

 the upper and lower zones, and do not pass into them. Mingled with the 

 tissue described are bands of parenchyma, or towards and at the center 

 are groups of parenchyma which retain the primitive condition. Besides 

 the two kinds of tissues described, and also in the middle zone, there are 

 true vessels which extend upward to the woody cylinder of the root (of the 

 parasite) and which also go downward through the lower zone, enter the 

 sinker, and pass through it to or near to its distal end. Some of the ves- 

 sels, perhaps all, are ultimately connected with the ducts of the host in a 

 peculiar way, to be described below. It is assumed that the tracheid-like 

 cells of the middle zone do not function as conductors of water, but serve 

 for mechanical purposes only, and may result from strains and stresses 

 which the cushion must undergo either with the growth of the root from 

 which it springs or that of the host. 



Between the main portion of the haustorium and its tip, the sinker, the 

 cells are mainly parenchymatous, with a few strands of ducts, usually iso- 

 lated, and possibly sieve-tissue. In the central portion the parenchyma is 

 large and intercellular spaces are prominent. Towards the periphery the 

 cells are small, and the intercellular spaces are small. The cells bounding 

 the sinker are either short where the absorption of the host is not active, 

 or long and narrow where there is the most apparent activity. As com- 

 pared with the cortical cells of the host, those of the sinker are richly sup- 

 plied with protoplasm and contain large nuclei. 



