HOW SUGAR IS TRANSFORMED INTO STARCH. 137 



3°. Without dwelling at present on this point, tlie above considera- 

 tions may be regarded as giving additional strength or probability to the 

 conclusions v/e formerly arrived at (p. 63) from other premises — that 

 the roots, besides carbonic acid, absorb certain other soluble organic 

 compounds, which are always present in the soil in greater or less 

 quantity, and that the plant appropriates and converts these into its own 

 substance. Some of these organic compounds may readily, and by ap- 

 parently simple changes, be transformed into the starch and woody fibre 

 of the living vegetable. The illustration of this fact will be reserved 

 until, in the second part of these lectures, I come to treat of the vegeta- 

 ble portion of soils, and of the ^chemical nature and constitution of the 

 organic compounds of which it consists, or to which it is capable of giv- 

 ing rise. 



4°. The chemical changes above explained (a), show how, from 

 carbonic acid and the elements of water, substances possessed of the 

 elementary constitution of sugar and gum may, by the natural processes 

 of vegetable life, obtain the elements of which they consist, and in the 

 requisite proportions. They throw no light, however, upon the me- 

 chanism by which these elements are constrained, as it were, to assume 

 first the form of gum or sugar, or soluble starch, and afterwards^ in 

 another part of the plant, of insoluble starch and woody fibre. 



It is known that the sap deposits starch and woody fibre in the stem, 

 only in its descent from the leaf, — and it is, therefore, inferred that the 

 action of light upon the sap, as it passes through the green parts, is ne- 

 cessary to dispose the elements to arrange themselves in the form of 

 vascular fibre or lignin. And as, by the agency of nitric acid, starch 

 appears to be convertible into woody fibre (p. 126), it is not unlikely 

 that the soluble substances, containing nitrogen, which are present in 

 the sap may — as diastase does upon starch — exercise an agency in trans- 

 forming the soluble sugar, gum, &c., of the sap into the insoluble starch 

 and woody fibre of the seed and the stem. We are here, however, upon 

 uncertain ground, and I refrain from advancing any further conjectures. 



Two great steps "we have now made. We have seen how the germ 

 lives and grows at the expense of the food stored up in the seed — and 

 how, when it has obtained roots and leaves, the plant is enabled to ex- 

 tract from the air and from the soil such materials as, in kind and quan- 

 tity, are fitted to build up its several parts during its future growth. 

 That considerable obscurity still rests on the details of what takes place 

 in the interior of the plant, does not detract from the value of what we 

 have already been able to ascertain. 



§ 3. On the production of oxalic acid in the leaves and stems of plants. 

 In the preceding section we have studied the origin of those sub- 

 stances only which form the chief bulk of the products of vegetation, 

 and which are characterized by a chemical constitution of such a kind 

 as enables them to be represented by carbon and water. But during 

 the stage of vegetable growth we are now considering, other compounds 

 totally different in their nature are also produced, and in some plants in 

 sufficient quantity to be deserving of a separate consideration. Such is 

 the case with oxalic acid. * 



The circumstances under whichf this acid occurs in nature have al- 



