514 MINNESOTA BOTANICAL STUDIES. 



the root in penetrating a loose substratum, and the relatively 

 high osmotic coefficient of the acid content of the storage cells. 

 As species of this genus exhibit a tendency to condense 

 reserve carbohydrates into solid form, and lower the acid 

 content, the mechanical elements increase in value. 



3. The product of the photosynthetic action of the leaves 

 is probably cane sugar, which in case of surplus accumula- 

 tion is converted into a form reacting reddish brown to iodine 

 solutions. During translocation it assumes the form of a cop- 

 per reducing substance, and accumulates in the tubers as 

 cane sugar. During the season of greatest chlorophyll ac- 

 tivity a portion of the cane sugar is condensed into the form 

 of "red starch" by the leucoplasts surrounding the nucleus. 

 The starch disappears on the formation of the propagative 

 shoots, in autumn. 



4. The tendency to form "red starch" is characteristic and 

 may not be altered by exposure to high proportions of carbon 

 dioxide, feeding with carbohydrates, or condensation by glyc- 

 erine. 



5. No connection could be traced between the distribution 

 of the mineral salts, except calcium, and the carbohydrates. 

 The presence of 1 per cent, of calcium or potassium nitrate in 

 a sugar solution will inhibit its condensation into starch. 



6. The tendency to form tubers on the roots seems firmly 

 fixed, and such formation occurred in water cultures in an ap- 

 parently starving condition. 



7. The sap of the external tissues contains a bitter tasting 

 substance and in addition the outer cortical cells of the roots 

 and tubers contain large drops of oil. These substances may 

 subserve as a means of protection, since no plants have been 

 observed to have been injured by 1 animals, and only in rare 

 instances has the investing mycelium penetrated the tuber, 

 by means of its haustorial branches, as far as the endoder- 

 mis. 



I am indebted to Mr. Percy Groom of Oxford, England, Mr. 

 Theo. Holm of Washington, D. C, Professor Geo. F. Atkinson 

 of Cornell University and others for helpful suggestions and 

 material during the course of the work, and to Miss J. E. 

 Tilden for the preparation of the plates. 



