176 SPIRAL VESSELS DUCTS, [BOOK n. 



conclusions, which, if not exactly, are probably substantially 

 correct : " That plants, like all other living bodies, require, 

 for the support of their vital functions, a free communication 

 with air ; and that it is more especially oxygen which when 

 absorbed by the roots from the soil, renders the crude fluid 

 fit for the nourishment and support of a plant, just as blood 

 is rendered fit for that of animals. But, for this purpose, it 

 is not sufficient that the external surface should be surrounded 

 by the atmosphere ; other aeriferous organs are provided, in 

 the form of spiral vessels, which are placed internally, and 

 convey air containing an unusual proportion of oxygen, which 

 is obtained through the root, by their own vital force, from 

 the earth and water. In a hundred parts of this air twenty- 

 seven to thirty parts are of oxygen, which is in part lost 

 during the day by the surface of plants under the direct 

 influence of the solar rays/' 



With such evidence of the aeriferous functions of the spiral 

 vessels, it will doubtless to many appear probable that this 

 question is settled, as far as spiral vessels, properly so called, 

 are concerned. Whether or not ducts have a different function 

 is uncertain ; it is probable, however, from the extreme thin- 

 ness of their sides, that they are really filled with fluid when 

 full grown, whatever may have been the case when they were 

 first generated. 



Link, who formerly considered trachenchym a part of the 

 aeriferous system, afterwards declared its function to be that 

 of conveying nutritious secretions. (Element, ed. 2. i. 188.) 

 He considered that opinion proved by certain plants which 

 he had grown in a solution of prussiate of potash, having had 

 their spiral vessels stained blue when afterwards grown in 

 sulphate of iron. 



In his most recent writings he speaks of the question as 

 follows : " It has always been a question, whether the spiral 

 vessels are air tubes, or whether they carry the juices for 

 nutrition ? I myself have twice changed my opinion regard- 

 ing it, because it was more my object to arrive at the truth 

 than to insist upon being right. Dr. Schleiden despatches 

 the question very quickly. He says, ' I found, almost without 

 exception, in all Cactacese, that the vessels, as they issued 



