Atkins — Osmotic Pressures of some Plant Organs. 469 



The above results seem to show that while large variations are met with 

 in the osmotic pressures of various fruits and of the same fruit at various 

 stages, tubers, on the contrary, vary but little in the same species, though 

 there is a considerable range of variation between various species. This 

 result was not unexpected ; for tubers exist under very uniform conditions in 

 nature, and usually possess a large store of starch or other reserve material 

 capable of being mobilized and of thus restoring equilibrium. In this they 

 differ from the leaves, wliich are organs for elaborating compounds with little 

 or no store-material. The latter are consequently very liable to fluctuation 

 in pressure, as shown in a former paper. (Dixon and Atkins, loc. cit.) 



The determination of the mean molecular weights frequently throws light 

 on the metabolism, a fall pointing to formation of starch or some other 

 insoluble material or possibly to exhaustion due to respiration. A rise, on the 

 other hand, indicates the conversion of colloids into soluble starches, mono- 

 and di-saccharides, or synthesis of some sugars by assimilation. 



Conclusions. 



1. As the result of over fifty measurements, the deduction may be made 

 that similar plant organs of the same species have approximately equal osmotic 

 pressures. (This does not apply to leaves, for which see Dixon and Atkins, 

 loc. cit.) 



2. The osmotic pressure of the solutes in the fruits examined varied from 

 about 6 to 30 atm., and the mean mol. wt. from 110 to 231. 



3. The pressure in the underground organs studied ranged from 6"5 to 

 18'7 atm., while the molecular weight of the solutes ranged from 101 to 394. 



4. The red stem of Rheum officinale, gave the lowest recorded mean 

 molecular weight, viz. 76, with an osmotic pressure of 6'52 atm. 



5. The tuber of Helianthus tuberosus gave the highest mean molecular 

 weight found, viz. 394. 



This paper is properly a continuation of a joint paper with Prof. H. H. 

 Dixon; and I have much pleasure in thanking him for his advice throughout. 



