504 MINNESOTA BOTANICAL STUDIES. 



substance by the acid content. Thus the state of firmness 

 and plumpness of the tubers offers no indication of the pres- 

 ence or absence of carbohydrates. 



So far as I have been able to examine other species of this 

 genus the amount of development of the secondary and tertiary 

 woody tissue in the storage organs is in proportion to the 

 tendency to convert the carbohydrates into solid form, and thus 

 decrease the turgidity of the parenchymatous cells. It is of 

 course to be admitted that other factors influence the develop- 

 ment of woody tissues in root formations, but in such tubers 

 as those of I. biternatum the mechanical strains to be borne 

 by the roots are very slight. 



Outwardly the tubers are more or less irregular globoid or 

 cylindrical thickenings of the roots which may attain a diame- 

 ter of 5 mm. or about three times the diameter of a normal 

 root and the thickening may extend a distance of 2. 5 cm. along 

 the root. The metamorphosis of a root into a tuber may begin 

 a few centimeters from the tip and a constant increase in size 

 takes place during the entire life of the root — one to three 

 years. On seedlings the thickening begins in 60 to 70 days 

 after germination of the seed, when only three or four foliage 

 leaves had appeared. The first outward indication of the 

 change is the glistening silvery white appearance of the por- 

 tion of the root concerned. 



In a brief description of the anatomy of the tubers Professor 

 C.W. Hargitt(V) has noted that the mass of the tuber was due to 

 the accented development of the conjunctive parenchyma, and 

 also concluded that these cells contained inulin and that the 

 "subepidermal" tissue contained aleurone. I have been unable 

 to confirm this diagnosis as to the reserve material, and must 

 also reject my former conclusion (XXII) that the tubers are 

 not storage organs, a conclusion to which I was led by the early 

 stage in the development of the root in which tuberous thick- 

 ening might begin, their behavior when free from surplus food 

 and the presence of a mycelium infecting the outer layers of 

 several lots of material examined. 



It has been determined that the presence of this organism is 

 purely incidental and in a few instances only has it penetrated 

 farther than the endodermis. 



Keserve material. In the work upon the character and se- 

 quence of the reserve substances from 1888 to 1893 only mater- 

 ial taken from the natural habitat under entirely natural con- 



