178 CTNENCHYM. AIR-CELLS. [BOOK n. 



flows from the Hoya carnosa, comes from the proper vessels, 

 sap vessels, the same as the milky juice in the Asclepiads. 

 These vessels, generally, however, have no partitions. If, 

 then, the nutritive sap made a rapid transition from the 

 spiral vessels into the cells (withered twigs, for instance, 

 placed in water, very quickly erect their leaves), would it be 

 seen ? But this is not the place for the investigation of this 

 subject ; it was only necessary to give Dr. Schleiden's state- 

 ment in his own words." (Ray Reports.} 



I cannot say that Professor Link meets the question in 

 a satisfactory manner : or at all shows, by what is very like 

 special pleading, that spiral vessels are not air vessels. It 

 has always appeared to me probable that one of the offices of 

 the spiral thread coiled in the interior of the tubes of 

 trachenchym, is to exclude fluids which would otherwise pass 

 through the thin sides, which object it can effect by the 

 mechanical obstacle opposed to fluids by the great additional 

 thickness the spiral thread gives to the sides. 



It requires no argument to prove that the office of CINEN- 

 CHYMA is to convey the elaborated sap of a plant to the places 

 where it is needed, and especially down the inner parts of 

 the bark of exogens. 



In regard to the functions of air-cells and lacuna, it may be 

 sufficient to remark, that in all cases in which they form a 

 part of the vital system, as in water plants, they are cavities 

 regularly built up of cellular tissue, and uniform in figure in 

 the same species ; in which case they serve to render parts 

 buoyant, or are in some other manner manifestly connected 

 with the peculiar mode of life of the plant in which they 

 occur ; while, on the other hand, where they are not essential 

 to vitality, as in the pith of the Walnut, the Rice-paper plant, 

 the stems of Umbellifers, and the like, they are ragged, 

 irregular distentions of the tissue. In the former case they 

 are intended to enable plants to float in water ; in the latter, 

 they are caused by the growth of one part more rapidly than 

 another. 



All these organs are held together by a skin or epidermis, 



