10 ROOT HABITS AND PARASITISM OF KRAMERIA CANESCENS. 



The distal ends of the haustoria usually comprise a group of two or more 

 sinkers, which penetrate into the wood of the host, and an epithelial layer 

 generally terminates these sinkers. But in many old haustoria the distal 

 cells of the sinkers have lost the epithelial character and are no longer 

 thin-walled, but are so modified that the walls are pitted and the contents 

 of the cells are lost. Further, these cells serve as links which unite the 

 adjacent vessels of the host to the ducts of the parasite, and thus the appa- 

 ratus of water-conduction from host to parasite is completed. The exact 

 manner in which the connection between parasite and host is brought 

 about was observed in a few instances, and the ducts of the host were 

 seen to be turned into the tip of the sinker for a short distance, and groups 

 of cells of the haustorimn encircle and terminate the ducts in the manner 

 indicated by fig. 4, plate 3. 



The history of the development of the tissues of the haustorimn, espe- 

 cially those of the sinker, shows that the parasite does not perfect its water 

 relations with the host until long after the woody cylinder of the host has 

 been reached and penetrated, and in some hosts this is apparently never 

 done. This condition is very different from that of the mistletoe, which 

 depends wholly on the host for its water-supply and which early connects 

 with the conductive system of the host- 



KRAMERIA AND ITS HOSTS. 

 Acacia Constricta. 



Acacia constricta is one of the smaller trees, frequently little more than 

 a shrub in the vicinity of Tucson, which is to be found along washes, on 

 the river-bottom, on favorable drainage-slopes, or wherever the soil has 

 depth and the water conditions are relatively good. The specimen espe- 

 cially studied and here described, which was attacked by Krameria, grew 

 near the wash west of Tumamoc Hill. The soil at this place is more than 

 2 meters deep and is a sandy loam. 



The root-system of Acacia is of the generalized type. There is a deeply 

 penetrating main root and several laterals which may extend as far as 

 4 meters from the central axis, and lie 40 cm. and deeper beneath the 

 surface. The roots attacked with the parasite were small ones which lay 

 near the surface of the ground. 



The parasitic relationship of Krameria and Acacia was demonstrated in 

 a root 3.5 mm. in diameter. In this instance the haustorimn had pene- 

 trated the cortex of the host to the ring of the mechanical tissue, the outer 

 edge of which is indicated in the figure by the line midway between the 

 cambium and the cork, along which it had extended for some distance. 

 The tip of the haustorium had reached through the ring and had attained 

 the woody cylinder of the host, into which it had penetrated slightly. 

 The material at hand was not favorable for a more extended examination 



