464 METABOLISM IN LIVING PLANTS. 



role may bo assigned to the organic acids of the type of oxalic and formic acids 

 with regard to the turgescence of cells in living plants, since they suck up water 

 with great enei'gy to replace that lost by evaporation, and are thus able to main- 

 tain the turgidity. 



An especial function is also assigned to the so-called amides, b}^ which are 

 understood asparagin, tyrosin, leucin, glutamin, &c. These are produced by the 

 splitting up of albumens, but at the same time they promote the reconstruction 

 of these substances in the living protoplasm. When the carbohydrate, which is 

 derived from the albumen, together with the amide, is used up, the amide again 

 draws to itself a fresh carbohydrate (which has just been formed in the green 

 cells), enters into combination with it, and in this way again forms an albumen. 

 This process may be repeated indefinitely, and will be referred to in the dis- 

 cussion on respiration. Moreover, when albumens, which, in their usual condition, 

 cannot pass through the cell-wall, are to be transmitted from one place to 

 another, they ai'e probably first transformed into asparagin, or a similar amide, 

 which again becomes a complete albuminous compound by the union with a 

 carbohydrate in the place where the albumens are to remain. 



Finally, the group of enzymes or ferments comes under the head of ac- 

 cessories. These substances, so extremely important to the life of plants, have 

 the remarkable property of being able to decompose other substances without 

 themselves being split up, and in consequence a very small quantity of them 

 can produce very marked results. They all contain nitrogen, and are widely 

 distributed in plants, but since they are only formed in minute quantities in the 

 places where they are required, their presence is not always easy to demonstrate. 

 How they arise is still a problem; perhaps in the same way as the nitrogenous 

 albumens. They are to be found wherever solid bodies are to be liquefied or 

 dissolved; for example, when the stores of organic food, i.e. the so-called reserve 

 matei'ials, which have remained resting for a long time in the seeds, tubers, 

 and roots, and have been, so to speak, put out of the way, are to be liquefied 

 and again bi-ought into action; further, when substances which cannot pass 

 through the cell-walls are to be brought into a condition suited to this translation, 

 in which case they act like the amides previously described; further still, as 

 often as organic compounds are to be absorbed as food, insects and other animals 

 to be digested by insectivorous plants, the dead bodies of plants to be broken 

 up by saprophytes, or the organized portions of living plants to be consumed by 

 parasites. When the sucking cells of the parasitic plants wish to obtain the 

 juices of the host-plant; when the hyphee issuing from the spores make their 

 way through the epidermis into the interior of the plant on which they have 

 fallen; or hyphal threads in the interior of a many-celled tissue wish to pass 

 from one chamber into another, they must dissolve the cell-walls, thus creating 

 an open passage for themselves. Enzymes also appear to come into action wherever 

 those remarkable processes are carried on which are known as fermentations, and 

 which will be considered in the following pages. It is to be supposed that they 



