HYGROSCOPICITY 



175 



3 or 4 and never over 5 per cent,, whatever the nature or size 

 of the pod. These pods, all of them dry, intact, and contain- 

 ing their seeds, range in length from 3 or 4 inches, as in the 

 case of the Pea, to a couj^le of feet, as with the pods of the 

 species of Cassia and Poinciana ; and there is a great contrast 

 between the relatively fragile appearance of the first-named 

 and the tough, woody aspect of the two last. However, not- 

 withstanding the great differences in size, weight, and texture 

 of these pods, their changes in weight due to the hygroscopic 

 reaction are not far apart. The long pods of Cassia fistula are 

 very sensitive to the hygrometric condition, and vary in weight 

 whilst being handled for the balance. One of them lost 

 I per cent, of its weight whilst a wet morning was giving 

 place to a fine evening. This amounts to a great deal in the 

 balance, since the pods when dry range between 1500 and 

 2000 grains in weight. They are durable, easily weighed, 

 and might prove useful as hygrometers. 



It would seem from the foregoing data that permeable 

 leguminous seeds possess much the same degree of hygro- 

 scopicity as their pods. Thus under the same conditions peas 

 gave a range of 4*5 per cent., and the pods with seeds removed 

 4-7 per cent. (The same principle is also indicated in the case Pods are 

 of Iris fcetidissima^ where the ranges for the seeds and the capsular vflK;ther°t?ie 

 valves were 4*0 and 7*6 per cent.) However, there is no seeds are 



^ . , , II- permeable or 



such relation between the impermeable seed and its pod. All impermeable. 



the impermeable seeds with which I am acquainted, such as 



those of Guilandina bonducella, Mucuna urens, Diodea reflexa^ 



Entada scandens^ etc., are enclosed in pods that in the dry state 



readily take up and absorb moisture ; whilst in the case of 



the hygroscopic pods of the species of Cassia, Poinciana, Entada, 



and Albizzia, dealt with in the table, many of the seeds are 



impermeable, and the hygroscopic range for any sample of 



seeds chosen at random is consequently low, usually not over 



I or 2 per cent., but varying according to the proportion of 



permeable seeds. As illustrating the hygroscopic behaviour 



of pods emptied of their impermeable seeds, I may mention 



