XXXI. 



SOME EXrERIMEXTS OX THE PHYSICS AND CHEMISTRY OF 



THE SAP OP PLANTS. 



By Peofessok Johk Attfielb, Ph.D., F.K.S., F.C.S., etc. 



Read at Watford, \lth April, 1883. 



How plants grow, whence comes their food, what is its nature, 

 and how it is conveyed, are questions which have interested man 

 in all ages. And to earnest, intelligent, unprejudiced truth-seeking 

 encpiirers answers to the questions have been obtained ; answers 

 general and incomplete at first, not even yet minute and perfect. 



Burn a log of wood in a grate with due access of air. The wood 

 and the oxygen of the air together yield carbonic- acid gas, vapour 

 of water, and nitrogenous gas, all of which pass up the chimney ; 

 ash containing various mineral matters, which falls beneath the 

 grate ; and heat, which warms and comforts us. Conversely nature 

 brings together heat, mineral matters, nitrogenous gas, water, and 

 carbonic acid gas, and the products are wood and oxygen. To 

 extract temporary comfort from the wood of our trees (or from coal, 

 which is only altered wood), we convert that wood into warmth, 

 smoke, and ash, and at the same time use up life-giving air. Bene- 

 ficent nature gathers together that warmth, and smoke and ash, 

 and returns them to us in the form of trees and flowers, at the 

 same time presenting us with the exact amount of life-giving air 

 we had lost. How do plants grow ? By additions of carbon, 

 nitrogen, the elements of water, and certain mineral elements. 

 Whence comes their food? From its storehouses, the atmosphere 

 and the earth. How is it conveyed ? By the carriers, air and water. 



But these answers to the questions respecting the growth of 

 plants, though very direct, are very crude and general. They 

 represent only the beginning and the end of a vast number of pro- 

 cesses. A log of wood on the one hand and the products of its 

 burning on the other are only the terminals of two long series of 

 substances, an analytical series beginning with the wood and ending 

 with gases and ashes, and a synthetical series beginning with gases 

 and ashes and ending with the wood, lieduce the wood to its 

 elements, not by the rough and rapid method of free combustion, 

 but by controlled heat in retorts, as at gas-works, and you will 

 obtain a series of substances which inehules scores of interesting 

 materials— acids, spirits, colour-yielding bodies, illuminating agents, 

 etc. Conversely, nature, between the constituents of the air or of 

 the soil on the one hand, and the finished tree or flower on the 

 other, forms a series of substances which includes scores of inter- 

 esting materials— acids, perfumes, colours, flavours, starch, sugar, 

 oil, etc. 



In these two series or chains the products are related to each 

 other or linked together, but in what way we do not yet perfectly 

 know. Indeed, as to the form or character of many of the links 



