178 BOTANY 



lime, a third common salt, while the aluminium, on the other hand, 

 is rejected alike by all three. The action of sea-weeds in this 

 respect is even more remarkable ; living in a medium containing 3 

 per cent of common salt, and but little potassium salts, they nevertheless 

 accumulate much larger quantities of potassium than sodium. In 

 addition they store up phosphates, nitrates, and iodine, — substances 

 which are all present in sea-water in such small quantities as scarcely 

 to be detected by chemical analysis. 



That osmosis may continue from cell to cell, it is essential that 

 the absorbed material must become transformed into something else, 

 either by the activity of the protoplasm or by some other means. 

 Local accumulations of sugar or other soluble reserve material in 

 fruits, seeds, bulbs, and tubers would otherwise not be possible ; for 

 osmotic action, if undisturbed, must in the end lead to the uniform 

 distribution of the diffusible substances equally throughout all the 

 cells. But if equilibrium is prevented by the transformation of the 

 diffusible substances into others that are indiffusible, the osmotic 

 currents towards the transforming cells will continue, and the altered 

 and no longer diffusible substances will be accumulated in them. In 

 this manner glucose passing into the cells of tubers or seeds becomes 

 converted into starch. As a result of this a constant movement of 

 new glucose is maintained towards these cells, which thus become 

 reservoirs of accumulated reserve material. 



From the power of protoplasm to regulate osmotic currents, in that 

 by reason of its permeability it allows the osmotic forces to operate, 

 or, on the other hand, may modify and altogether prevent them, it is 

 apparent that here also, just as in the case of the rigidity of plants, 

 osmosis, although a purely physical phenomenon, is controlled by the 

 protoplasm and rendered serviceable to plant life. 



Water and Mineral Substances 



The fact that water is essential to the life of all living organisms is 

 so obvious that, in the infancy of natural history and philosophy, from 

 Thales to Empedocles, water was regarded as the original principle 

 of all existence, at least of the organic world. Even so late as the six- 

 teenth century it was held by Van Helmont, the first to investigate 

 experimentally the question of the nutrition of plants, that the whole 

 substance of plants was formed of water. If the importance of water 

 in this respect was greatly overrated, the universal necessity of water 

 for all vital processes is still recognised in the present more advanced 

 stage of scientific knowledge. Without water there can be no life. 

 The living portions of all organisms are permeated with 

 WATER ; it is only when in this condition that their vital processes 

 can be carried on. Protoplasm, the real vehicle of life, is, when living, 



