102 
Peirce. — A Contribution to the 
tion of potato-tubers by the rhizomes of ‘ Quitch Grass ’ 
( Ag ropy rum repens , Beauv.). From experiments of my own 
on seedlings of various plants producing both large and 
small main roots, it is perfectly evident that the power of 
penetrating living tissues by pressure alone is not a peculiarity 
of haustoria (which are but lateral roots modified in origin and 
structure to accomplish a very special purpose), nor of the 
roots of some Grasses. If a root be so firmly fixed against 
a plant that, if it grows at all, it can only grow into the 
opposing tissues, it will invariably penetrate them. Into how 
solid tissues these ordinary roots can make their way it is not 
within the scope of this paper to discuss. It is sufficient now 
to state that they readily penetrate through tissues as solid as 
the cortical tissues of any of the hosts of Cuscuta which I have 
yet seen. As recently determined by Pfeffer 1 , the pressures 
which ordinary roots are able to exert are great. Accurate 
determinations of the pressures exerted by haustoria are, on 
account of the habits of the plants, most difficult to make ; 
but from the celerity and apparent ease with which the 
haustoria of C. glomerata penetrate tin-foil it is hard to 
suppose that they are the weakest of all roots. 
From the foregoing experiment it becomes evident that, 
though one object of the close coils formed about its host 
is to produce intimate contact of a considerable area of the 
parasite with its host, in order to induce the formation of 
numerous haustoria, there is still another and also a very 
important object. Owing to the closeness of the coils and 
to the large area consequently applied to the host, any force 
acting between the parasite and the host and tending to push 
them apart, would be resisted by the very great friction which 
must be overcome before the parasite could be made to uncoil. 
This resistance is not far from the breaking-strength of the 
stem. The force developed by the growing haustoria tends 
to push host and parasite apart, and it is resisted in this way. 
But the matter is not so simple. After a branch has made 
1 Pfeffer, W., Druck- und Arbeitsleistung durch wachsende Pflanzen. Abhandl. 
d. K. S. Gesellschaft d. Wissenschaften, XXXIII, 1893. 
