39 



M. Mohl has published a memoir on the structure of the 

 porous vessels of dicotyledons*, and I have also treated of this 

 subject in the fifth chapter of my work, but I have called these 

 porous vessels dotted spiral tubes. 



M. Mohl adopts two varieties of dotted spiral tubes ; in the 

 one the walls are lined equally on both sides with dots or 

 pores, according to M. MohPs statement ; the oak, alder, &c., 

 offer examples ; while in the other variety the tubes exhibit 

 a totally different structure at various parts, as in the lime, 

 the Italian poplar, and in many other woods. In the lime, the 

 walls of these ducts, which abut on the ligneous cells, have all 

 the appearance of spiral tubes capable of unrolling, while the 

 other walls, by means of which these vessels cohere among 

 themselves, exhibit the series of dots, which are always situated 

 between two convolutions of spiral fibre. It is evident, there- 

 fore, from these observations, says M. Mohl, that the dotted 

 spiral ducts belong to the system of spiral tubes, and the most 

 essential part of their formation consists in this : that between 

 the coils of the spiral fibre there is a dilated membrane, on 

 which, between every two fibres, a series of dots is situated. 

 According to my view all the coils of spiral fibres are clothed 

 with a fine membrane, and the coils of fibre take some part in 

 the formation of the dots by reciprocal cohesion. In proof 

 that the dotted spiral tubes belong to the system of true spi- 

 ral tubes, I have mentioned a case in the stem of a gourd, 

 where at times the large spiral tubes are not metamorphosed 

 into dotted tubes, which in this plant is otherwise very fre- 

 quently the case. 



M. Mohl does not consider it as improbable that the thick- 

 ening of the membrane of spiral vessels may be effected by the 

 deposition of new layers upon their inner surface, exactly as 

 with the thickening of cellular membrane ; and I have actually 

 observed this in several cases, representations of which are 

 given in my Vegetable Physiology, plate III. figs. 15 and 16. 



M. Mohl and I also agree in the explanation of the struc- 

 ture of the dots, namely, that they are formed precisely in 

 the same way as the large dots of coniferous and cycadeous 

 wood ; this indeed could not but be expected with the use of 



* Abhandlungen der mailiem. physik. Klasse der Akad. der Wissensch. 

 zu Munchen, 1837, i. p. 415. 



