40 MINNESOTA BOTANICAL STUDIES. 



fungi, and algae grow within or upon the tissues of other plants, 

 to which the product of their assimilation of free nitrogen are 

 directly available. Symbioses of the latter form are exhibited 

 in the tubercles of the leguminous and a few other plants, by 

 the endophytic mycorhiza of a large number of herbaceous 

 plants and the epiphytic mycorhiza of forest trees. And at the 

 present time more than one hundred species of algae^ are 

 known to be found within the tissues of other plants with which 

 they sustain certain mutual relations. 



Many additions to these forms of mutualism and symbiosis 

 may be expected, while the nature of the interchanges which 

 take place between the higher and lower plants have not been 

 made out in any case, with any degree of accuracy. 



For several years the author has had under observation 

 Isopyrum biternatum, a small plant of the Ranunculaceae, 

 which inhabits North America northward from Kentucky, and 

 eastward from the Rocky mountains. Attention was called to 

 the tuberous thickenings of the fibrous roots, by Dr. O. P. Jen- 

 kins, in 1888, and they were found to exhibit such peculiar 

 features of structure and behavior as to be only explainable in 

 the light of recent research on the assimilation of nitrogen in 

 the higher plants. 



These tubers are apparently a constant feature of the roots, 

 since they are mentioned in nearly all systematic works con- 

 taining descriptions of the roots and were present in all speci- 

 mens examined since 1888. In the plants which have come 

 under observation the tubers are present in all stages from 

 barely perceptible swellings to irregular cylindrical, or spindle- 

 shaped enlargements, 6mm. in diameter, and 2cm. in length. 

 As many as 30 or 40 may be found on a single plant representing 

 a total volume of 1 — 2 cu. cms. These tubers are formed con- 

 temporaneously with, or previously to the secondary thicken- 

 ing of the roots, and show a glistening silvery- white surface in 

 contrast to the normal brown color of the roots. The '' Silher- 

 glanz" is apparent even at the beginning of their formation, 

 and by this appearance the tuber-forming portion of the root 

 may be known before any enlargement has taken place. With 

 age, however, the tuber takes on a brownish tinge. 



The structure of the normal root is typical of this group of 

 the Ranunculaceae. In the formation of the tubers, the camb- 

 ium at points opposite two or sometimes three of the xylem 



1. Moebius. Conspectus algarum endophyticum. La Notarisia 6 : 1221. 1279. 1291. 

 1803. 



