DEPARTMENT OF BOTANICAL RESEARCH. 



51 



Growth and the Accumulation of Reserve Material as measured in the Potato, 



by D. T. MacDougal. 



The potato tuber is an enlargement of a portion of an underground 

 branch of the plant. Its development and increase in size are due 

 chiefly to the multiplication of thin-walled cells of the cortex and 

 medulla, and accumulating carbohydrates are condensed and stored as 

 starch in these cells during growth. In this accumulation of sohd 

 material in the cells the tuber offers contrasts with any other organs 

 previously studied. 



The course of growth in such structures may be exemplified by a 

 summary of the record of Tuber No. 2 which was measured August 10 

 to September 26, 1919. The weekly increase in diameter as recorded 

 directly by the auxograph and the calculated increase in volume may 

 be compared in the following table : 



It is to be seen that the tiTie measure of growth of such organs is to 

 be found in changes in volume. The two cycles of enlargement are 

 apparent. Fifteen competing tubers on other branches reduced the 

 supply of water and material coming to this tuber, with the result that 

 it was brought to a standstill. No further development would have 

 ensued had the competition not been removed. When this was done 

 enlargement was resumed and continued for four weeks, at the end of 

 which time a diameter of about 2 cm. had been reached. 



Tubers, like all organs the growth of which has been studied, show a 

 daily variation controlled by water-supply and water-loss. Actual 

 enlargement is checked or canceled during the midday period, when 

 transportation reaches the highest rate, and during the period of great- 

 est water-deficit by the shoot. 



The development of a number of tubers furnished confirmation of 

 these and other features which will be described more fully in a tech- 

 nical paper in preparation. 



Physical Factors in Growth as determined in the Tomato, by D. T. MacDougal.^ 



The author has previously pointed out that the tissues of animals 

 and of the greater number of plants accumulate carbohydrates, pro- 

 teins, salts, and other solids during the course of growth in such manner 

 that the relative dry weight of an organ is least in the embryonic or 



1 From the Bulletin of the Torrey Botanical Club, vol. 47, 261-269 (1920). 



