Experiments on Plants 781 



and that different groups of algae correspond to different levels. 

 Here, it is not to warmth but to light that changes in the bathy- 

 metric distribution are usually attributed. The problem is also 

 complicated by the varying content of free oxygen and carbonic 

 acid in the depths. But besides these conditions ordinarily con- 

 sidered, we must not forget the pressure, which may perhaps have 

 some effect. 



We realize that these questions cannot be solved by direct ob- 

 servation and that they demand the assistance of experimentation. 

 But we also understand that it is extremely difficult to carry on 

 such experiments. Plants do not betray by immediate signs, as ani- 

 mals do, the painful effects of new conditions. They must be kept 

 under these conditions for a long time to give results. Besides, to 

 live they must have light. Glass apparatuses capable of sustaining 

 decreased pressures are difficult and costly to install. It is still 

 more difficult when we wish to use increased pressure; the small 

 dimensions, the thickness of the glass, the use of cast-iron pieces 

 and protecting grillwork make it almost impossible to carry on ex- 

 periments under satisfactory conditions. 



There is, however, one plant phenomenon which lends itself to 

 experimentation at different pressures, because it takes place in 

 darkness and requires little room; that is germination. It is to 

 such experiments that I have devoted myself almost exclusively. 



For vegetation properly so-called, I have often used sensitive 

 plants. This valuable plant was used thus as a reagent, as a kind 

 of warm-blooded plant, as I once called it. 1 



In my bibliographical research, made earlier at the end of my 

 experiments when I was beginning to write the present volume, I 

 found that an early experimenter had already investigated this 

 question, and I quote here his account of his attempt to settle it. 2 



To study the influence of different pressures of the air on vegeta- 

 tion, or rather on the size and form of plants, at the same time I 

 sprouted barley in air rarefied by one half, in which the barometer 

 stood at 14 inches, and in air compressed to double the ordinary 

 pressure, that is, a pressure of 2 x 28 = 56 inches of mercury. In both 

 experiments the seeds were sown in heather compost and equally 

 moistened. Each of the two bells in which the germinating process 

 took place contained about 320 cubic inches of air, and consequently 

 the first contained 320 = 160 cubic inches of atmospheric air, and the 



2 

 second 320 x 2 = 640 cubic inches. 



The germination of the barley took place at about the same time 

 in both receivers, and the budding leaflets showed about the same 



