14 4f Circulating Syste7n of Plants. 



Mr. Knight had long since proved that the specific gravity 

 of the sap of trees increases in the spring, in proportion to its 

 distance from the ground ; and that saccharine matter is formed 

 at that season in the alburnum (the young sap-wood) of trees 

 which contained none in the winter. From these and other 

 facts, Mr. Knight arrived at the conclusion, " that it is 

 through the cellular substance of the alburnum, and not 

 through its tubes, that the sap ascends." 



The celebrated French philosopher, M. Dutrochet, visited 

 Mr. Knight at Downton, and spent nearly three weeks with 

 him. Before they parted, the opinions of the two gentlemen 

 became perfectly in unison. " We both agree," Mr. Knight 

 says, " that the water and nutriment absorbed from the soil 

 ascend through the cellular substance of the alburnum, and 

 pass through vessels, cellular in structure, which surround the 

 bundles of spiral tubes ; that the nutriment absorbed becomes 

 the true sap, or living blood, of the plant, by exposure to light, 

 in the leaf; that it descends by the bark (wherever plants 

 have bark), by which the matter that forms the layer of albur- 

 num is deposited ; and that whatever portion of the true sap 

 is not expended, sinks into the alburnum through the mis- 

 named medullary processes, and joins the ascending current. 

 As autumn, however, approaches, the expenditure of sap 

 diminishes, and it then accumulates in the alburnum, to be 

 employed in forming the young shoots and leaves of the ensu- 

 ing spring. I am in possession of a thousand facts to support 

 this hypothesis, and not in possession of one fact in opposition 

 to it." 



From another very recent communication with which 

 Mr. Knight has honoured me, in consequence of my urging 

 the assured fact that fluids are at times discoverable in vessels 

 of a tubular structure, I select the following passage, because 

 it tends to throw light upon, as well as to confirm, the theory 

 just adduced : — 



" The tubes of the sap-wood (alburnum) are often, in the 

 spring, quite full of sap ; and trees then, such as the vine, the 

 sycamore, and the birch, bleed. These may be called sap- 

 vessels ; but they are not the vessels through which the sap 

 rises ; because it will rise, and freely too, when all those are 

 intercepted ; and in the middle of summer, when the sap is 

 rising most rapidly, in dry hot weather, those tubes are always 

 dry and empty. They are reservoirs, which fill before the 

 leaves are prepared to throw off the aqueous part of the sap 

 which has ascended. 



" It is most certainly through the cellular substance that 

 the sap ascends. 



