410 MEYKN*S REPORT FOR 1837 OX THE 



error into which M. Mandl fell, in mistaking the minute 

 spiculae present in such sap for animalcules. 



The section of the report devoted to Vegetable Anatomy 

 is extremely ample, and contains a most valuable amount of 

 information. One or two extracts from the most novel por- 

 tions will at once afford a specimen of the lucid manner in 

 which this subject is treated, and make known some of the 

 most interesting discoveries to our readers. 



" M. Mohl has published a memoir on the structure of the porous ves- 

 sels of dicotyledons, and I have also treated of this subject in my fifth 

 chapter, but I have called these porous vessels dotted spiral tubes. 



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

 w^alls are lined equally on both sides with dots or pores, according to 

 M. Mohl's statement ; the oak, ald^r, &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 ap- 

 pearance 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, therefore, 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 spiral 

 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 frequently the case. 



" M. Mohl does not consider it as improbable that the thickening 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, representa- 

 tions of which are given in my Vegetable Physiology, PI. III. figs. 15, 16. 



" M. Mohl and I also agree in the explanation of the structure 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 such perfect instruments ; for most of the incor- 

 rect observations of former times can only be ascribed to the defective 

 microscopes of that period. 



" M. Mohl compares the development of the porous vessels with that 

 of cells, as series of thin-sided cell-like cavities constitute their base, in 

 which the spiral fibres are then formed. M. de Mirbel had already started 

 a similar notion, that vessels are formed from cells, and the observation of 

 the porous tubes in the earliest stages of their development is said to 

 prove this. About this time the individual cells are frequently found per- 

 fectly closed, and the thin membranous diagonal partitions subsequently 

 disappear, while they remain in many cases during the whole lifetime of 

 the plant, but take a structure quite different from that of the lateral par- 

 titions, which has already been demonstrated in various plants. I am 

 well acquainted with the cases which might lead observers to the above 

 views, but I also know of numerous cases in which the very opposite may 

 be observed, where both the simple as well as the metamorphosed con- 



