160 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



found that in the ripening of Citrus medico, the osmotic 

 pressure was reduced to almost exactly half of its original 

 value. Since during the process the fruit increased in size 

 from 14 to 27 millimetres in diameter during about six 

 weeks, it is evident that this fall in pressure is due to dilu- 

 tion of the solutes; for the hydrolytic changes in progress 

 would normally result in an increase of pressure, as may 

 be seen from the examples which follow. 



Cavara also traced the maturation of fruits of Pyrus 

 communis(^ 1-031 2-460), Vitis vinifera (A 0-710 2-516), 

 Opuntia piccolominea (A 0-460 2-080), and 0. ficus-indica 

 (A 0-521 2-360). These results are shown in the graphs 

 (Fig. 18). In each case a very large and sudden rise occurs 

 as the fruit reaches the final stages of ripening. 



SUBTERRANEAN ORGANS. 



It has been found that the osmotic pressure of roots, 

 rhizomes, tubers, and bulbs, is invariably lower than that 

 of the leaves of the same plant gathered at the same time. 

 To this Dixon and Atkins (1912, 2) found only one ex- 

 ception out of very numerous determinations, and it is by 

 no means sure that this measurement was reliable ; for the 

 roots and leaves were not treated with liquid air, conse- 

 quently the sap pressed from them was in each case too 

 dilute. The table on p. 161 shows the magnitude of the 

 pressures and electrical conductivities found. All deter- 

 minations were made with material which had previously been 

 treated with liquid air, unless where the contrary is stated. 



Inspection of Table XL. shows that even in under- 

 ground organs very considerable osmotic pressures may be 

 encountered. More especially is this true of storage 

 organs, such as the roots of Beta and Brassica, the tuber 

 of Helianthus, and the bulb of Allium, in all of which soluble 

 carbohydrates accumulate. In these plants it may be 



