104 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



presence of sunlight that this decomposition takes place, for in the dark, 

 or under the influence of a low temperature, the opposite process goes on, 

 the cell-contents undergoing oxidation to some extent and carbonic acid 

 being expired. This action, however, proceeds to a much less degree 

 than does that of carbon assimilation and the giving out of oxygen. It is 

 shown in the sprouting of the potato when kept in the dark, no green 

 parts being formed, and the starch of the potato undergoing oxidation ; 

 also in the case of parasitic piants like dodder and broom-rape, and with 

 all the fungi, plants which pcssess no green parts, i.e. which have no 

 chlorophyll. The former live upon juices of other plants on which they 

 thrive, while the fungi derive their nourishment from decaying organic 

 matter. 



The presence of oxygen in the atmosphere plays an important part, 

 further, in the germination of seeds. The seed contains the embryo, and 

 this is rich in albuminoids, fats, and mineral bodies (chiefly potash and 

 phosphoric acid). Under the influence of suitable moisture and tem- 

 perature the oxygen of the air penetrating into the soil reaches the seed 

 and oxidises the contents of the embryo, rendering them soluble, probably 

 by the agency of ferments present and known as "enzymes," when, as 

 the result of the oxidation, carbonic acid is given off. Hence atmospheric 

 oxygen is a necessity for germination to proceed properly. This is well 

 seen in the fact that seed, if buried too deeply in the soil, will not 

 germinate ; for, if placed beyond the depth to which the oxygen will 

 penetrate, the changes just described, and which result in the formation 

 of root and plumule, cannot proceed. 



In addition to taking in carbonic acid as described, plants have a 

 limited power of taking up ammonia through their leaves. Ammonia is 

 always present in small quantity in the air, but not nearly in quantity 

 sufficient to provide the plant's requirements of nitrogen. Nor is there 

 any evidence that plants have the power of taking up by their leaves or 

 other green parts the nitrogen of the atmosphere. Indeed, the exact 

 contrary has been proved by the classic work of Lawes, Gilbert, and 

 Pugh. Far, however, from the nitrogen of the atmosphere being a 

 useless ingredient in plant-life, recent investigations, commencing with 

 the discoveries of Hellriegel, have gone to open up a most interesting 

 field of inquiry, and to show that certain classes of leguminous plants 

 have, under favourable conditions, the power of utilising the nitrogen of 

 the atmosphere, not by their leaves, but by their roots. 



On the roots of leguminous plants of the order Papilionacere have 

 been observed swellings, or "nodules " as they are called, and examination 

 of these has shown them to be of bacterial origin and to contain 

 organisms which have the power of assimilating the atmospheric nitrogen 

 distributed throughout the soil, and of building it up to form the nitro- 

 genous constituents of these plants. It was found by Hellriegel that 

 when these nodules were present the plant throve and contained nitrogen 

 far in excess of anything capable of being accounted for by the nitrogen 

 in the seed, the soil, the water, or the ammonia of the atmosphere, while in 

 the absence of nodules the nitrogen and consequent growth were bounded by 

 the amounts supplied from these other sources. Also that when nitrogen 

 was withheld only a stunted growth was obtained and no nodules were 



