BOT.-VOL. II.] PEIRCE— ROOT-TUBERCLES. 313 



nor numerous but adequate for a time. A lateral root 

 grows both at the tip and throughout its length, thickening 

 and elongating. In this way its cylindrical form is main- 

 tained. The lateral root contributes food-materials and 

 water in increasing amounts to the plant which forms it. 

 The tubercle receives food from the plant. Perhaps it 

 contributes also to the nutrition of the plant. Experiment 

 so far seems to justify this belief. But if the tubercle were 

 altogether beneficial and increasingly so, one would suppose 

 that it would grow at the base, by secondary thickening, as 

 well as at the tip, by primary growth, in order through 

 increasing conducting tissues to contribute more and more 

 to the nutrition of the plant. The absence of such secondary 

 growth and the ultimate fate of the tubercle— dying and being 



cut off suggest that the leguminous plant limits as far as 



possible the supply of food to the tubercle, and finally stops 

 it. Herein we have another item of evidence against Frank's 

 hypothesis that the leguminous plant encourages tubercle 

 formation. It does not cut off the tubercle immediately; 

 the irritation which results in tubercle formation is too 

 great and the osmotic demand for food is too strong to be 

 resisted at once by the plant. Only after a time is tubercle 

 growth checked — perhaps by the remoteness of the tubercle 

 meristem from the source of food-supply — and later still, 

 the tubercle is cut off. 



The tubercles may be rosy pink at and near the tip, 

 creamy white elsewhere, nearly or quite the same shade 

 as the roots bearing them. Later, the oldest tubercles, 

 those nearest the surface of the soil, may branch, taking on 

 a flat, though thick, fan shape. By no means do all of the 

 tubercles branch. Those very near or almost on the surface 

 of the soil do not, and in the lower half of the infected 

 portion of the root-system I have seen almost no branched 

 tubercles. The difference in age between the branched and 

 unbranched tubercles in the same or the adjacent layers of 

 soil is not sufficient to account for the difference in form. 

 The branched tubercles are the first ones to lose the plump 

 and healthy appearance of active life ; they grow thin and 



