290 LECTURE XVII. 



bolism which occurs during growth, etc. In this connection, for example, lime often 

 plays a similar part to silica : just as numerous plants have their cell-walls silicified, 

 others may be calcified by the deposition of large quantities of carbonate of lime, 

 and thus become stone-hard, as the Melobesiacese and Corallineas among the Algse ; 

 and there is scarcely room for doubt that all, or at any rate very many older cell- 

 walls leave behind on combustion an ash containing lime, and occasionally, as in the 

 vessels of the Gourd, it is even possible to obtain chalk skeletons by combustion. 



Without -wishing to enter more in detail into the thousand-fold investigated 

 ashes of plants, it will still be well to bring forward two facts concerning them. 

 In the first place, every part of a plant, every cell-wall, even the youngest, and 

 likewise the protoplasm, and even starch-grains, leave an ash behind after com- 

 bustion; and from their general occurrence it may be concluded that certain 

 constituents of the ash at least are indispensable for the chemical composition 

 or for the molecular structure of the cell-walls and protoplasm. A second fact 

 worth noting is the abundance of ash-constituents in the green assimilating leaves : 

 this is at once explained when we know that these constituents are continually 

 being carried by the transpiration-current to the leaves, and are retained in them 

 on the evaporation of the water, and when we observe that no assimilation occurs 

 in their absence, as experiments on vegetation demonstrate. 



The quantity of ash, however, as a rule, amounts only to a small proportion 

 of the plant-substance, reckoned without water. It usually varies between lo/o and 

 10%; the wood and seeds belonging to the parts poorest in ash — the former evidently 

 because the ash constituents do not need to be accumulated in it for the purposes 

 of metabolism, but are conveyed to the organs of assimilation. The poverty 

 of seeds in ash-constituents, on the other hand, is explained on reflecting 

 that the seeds take up water laden with mineral matters when they germinate. 

 Finally, ash analyses show that the quantitative composition — i.e. the relative 

 richness in potassium, calcium, magnesium, and phosphorus — changes according 

 to the nature and the age of the organ of a plant. Ripe seeds, for example, 

 usually contain relatively much potassium, magnesium, and phosphorus. Since, 

 however, it is not possible to bring the facts so far known as to the composition 

 of the ash into immediate connection with definite physiological functions, we will 

 now leave this subject for the present. 



Turning once more to our water-cultures, we have now to take into con- 

 sideration another most important question. Our plants, as we have already seen, 

 consist only to a very small extent ,of the substances which were dissolved in the 

 water, and which have been absorbed by them. In the main, the substance of 

 our experimental plant consists, like that of any other plant, of cellulose, proteid 

 substances, small quantities of fat, and a few other organic matters. Besides 

 nitrogen and sulphur, which are contained in the proteid substances of the 

 protoplasm, the latter, as well as the cellulose and all organic substances employed 

 for the normal construction of the plant, contain hydrogen, oxygen, and carbon. 

 The source of these elements of the organic substance, so far as the hydrogen 

 and oxygen are concerned, presents no difiiculties whatever : these are in fact the 

 elements of water, which the plant absorbs in such enormous quantities, and which 

 permeates all parts of it, however small. The sulphur in the proteid substances, 



