220 



CASSELL'S POPULAK GARDENING. 



the other half be allowed to remain on the plant all 

 the day so as to he exposed to the sun, and be tested 

 at nightfall, abundance of starch will then be found. 



It would seem, therefore, at first sight that the 

 formation, or at any rate the action, of diastase in 

 dissolving the starch must take place at night only ;' 

 and this would be consistent with the now well- 

 established fact that new growth (not mere exten- 

 sion of old) takes place principally at night, or in 

 the dark. The change from the insoluble starch to 

 the soluble glucose goes on, however, during the 

 day as well as at night, though these changes are 

 less evident because during the day more starch is 

 formed than is abstracted. Sachs has endeavoured 

 to estimate the proportionate amount of starch pro- 

 duced in a given area of leaf -surface. By combining 

 his experimental results, and taking note of all the 

 circumstances, he " concludes that twenty to twenty- 

 fivB grammes of starch per day may be produced by 

 one square metre of leaf -surface as an ordinary oc- 

 currence." (One gramme is equal to 15 grains ; one 

 square metre, to 1-09 square yard.) 



It is cle'ar that some very important practical 

 deductions may be drawn from these facts. If the 

 amount of starchy contents of leaves varies during 

 the day, and still more so at night, the period at 

 which such leaves are cut for food or forage becomes 

 a matter of great importance. The afternoon or 

 evening would- thus be indicated as the best periods 

 for cutting vegetables for salad, for cooking, or for 

 medicinal purposes, so as to secure the produce of 

 the day before it is drafted off elsewhere. 



The rapidity of production of the starch depends 

 upon the intensity of light and the quantity of car- 

 bonic acid gas, the production being much more rapid 

 in intense light, and when the carbonic acid gas is in 

 large proportion. If there be no carbonic acid gas 

 at all, no starch is produced, even if the leaf be fully 

 ex,posed to the sun ; moreover, any starch that had 

 been formed previously disappears as it would do if 

 the leaf were kept in the dark. Starch is a ternary 

 combination of carbon, hydrogen, and oxygen ; and 

 by combining with the nitrogen, absorbed by the 

 roots as ammonia or nitrates, it forms the albuminoid 

 substances like protoplasm, the cell nucleus, &c., 

 containing varying proportions of carbon, hydrogen, 

 oxygen, and nitrogen. On the other hand, by the 

 progressively increasing loss of oxygen, it may form 

 some of the vegetable acids, and ultimately of the 

 hydro-carbons, such as resins, in which oxygen is 

 entirely absent. The cellulose of the oeU-wall is 

 formed, according to some observers, directly from 

 starch or sugar, of which it has the same chemical 

 composition, while others consider it to be formed 

 from the protoplasm. Gum, mucilage, arid various 

 substances of that character, are direct transforma- 



tions from cellulose, as probably are the jelly-like 

 substances in fruits. Chemists must, however, ad- 

 vance this portion of their science materially before 

 it can be made available for practical purposes. 



The Iieaf as an Organ of Digestion.— The 



starch alluded to in the last paragraph is insoluble. 

 It is requisite for it to be removed from the factory, 

 where it would be in the way and where it would 

 prevent the continued formation of fresh supplies, 

 and that it be stored up in the warehouse for future 

 use if not wanted immediately. To efiect this it must, 

 be dissolved, taken out of the leaf along the leaf -stalk 

 (where present), and thus conducted to the place 

 of consumption or to the bark, the seed, the tuber, 

 or other reserve organ, as explained in the earlier 

 chapters. This solution of the starch is effected by 

 a nitrogenous substance formed in the protoplasm, 

 and called " diastase." Why the diastase dissolves 

 starch it is impossible to say; but, under certain 

 conditions, of which the presence of an acid is one, 

 it does so, converts the starch into dextrose, and 

 afterwards into glucose or Grape-sugar. Thus, in 

 the germination of seeds, the starch in the white 

 perisperm is dissolved by the diastase in the embryo, 

 converted into sugar, and in this form is absorbed 

 by the embryo, and used up in the formation of the 

 growing tissues. The same thing happens when a 

 bud commences to grow, or a Potato to sprout. ' The 

 solution of the starch in bread is effected by a 

 similar ferment in the saliva. 



This process of digestion is apparently imiversal 

 in plants, but subject to various modifications, one 

 of the most striking of which is that afforded by 

 the Dioniea, the Drosera, and other so-called car- 

 nivorous plants. In these plants, when an insect or 

 any substance containing nitrogen comes into con- 

 tact with the leaf, an acid fluid Is poured out by the 

 leaf, which effects the solution of the insect just as 

 the gastric juice dissolves matters in the stomach, 

 and puts them into a fit condition to be absorbed. 

 In addition to diastase there are other ferments 

 formed in the cells, which effect the conversion of 

 starch to sugar, and the production of alcohol from 

 the latter, as in the yeast fungus. The reason why 

 some fungi effect this or corresponding changes, 

 while others do not, is that the former — e.g., Peni- 

 cillium — contain a ferment, while the latter — e.g., 

 Mucor — do not (Van Tieghem). " Pepsin," found in 

 the leaves of so-called carnivorous plants, effects the 

 decomposition of albuminoid matters, and converts 

 them into peptones. Its presence in the juice of the 

 Papaw-tree or Fig explains the fact that meat 

 wrapped in the leaves of these plants becomes tender. 



Fatty matters, which are often stored up in Jjlace 

 of starch, are acted upon in a similar manner by a 



