334 Meyen's Report for 1839 on Physiological Botany. 



Between the vessels there are compact masses of cells, 

 which also reach as far as the zone ; these separate, and a new 

 cellular tissue, whose sides are covered with papillae, insinuates 

 itself between them. It extends itself in the form of an irre- 

 gular layer, in the same direction as the compound vessels, 

 viz. towards the centre. 



M. Mirbel does not agree with those physiologists who hold 

 these cellular layers to consist of laticiferous vessels, but con- 

 siders them as lengthened cells destitute of coloured sap ; and 

 he thinks he has observed the gradual metamorphosis of the 

 utriculi into variously sized scalariform vessels. Each new layer, 

 in lengthening, separates the mass of tubes in the centre. 



In the mean time a new layer of cambium is formed in 

 each half, which is soon converted into a layer of cells and 

 then into vessels. These formations continue so long as cam- 

 bium is produced. 



The metamorphosis of cells and vessels from simple ones 

 into compound, takes place in the same manner as was stated 

 above, by means of the development of other cells in their 

 interior. 



I have given the contents of this very excellent work with- 

 out any remarks ; but I must add that I by no means agree 

 with all the results contained in it ; for on an examination 

 of the young roots of the date-palm I took quite a different 

 view of several of the above observations. I cannot, how- 

 ever, here enter further into the subject. 



M. Mirbel, considered them as small holes, and the latter seemed to assume 

 the presence of such holes in the membrane of cells as a general occurrence. 

 The Germans, in general, disputed the existence of these holes, but after- 

 wards acknowledged their error, and held these formations as true holes. 

 At a later period it was found that these pores were only thinner portions of 

 the membrane, which could be clearly seen with a good instrument. M. Mohl 

 described them as such. These differences of opinion arose solely from the 

 imperfections of microscopes ; but now we can always determine whether 

 at any spot there are holes or only thinner parts, and we must therefore mo- 

 dify our opinions on this subject. The small pores appear very generally as 

 transparent dots ; but we may easily convince ourselves that actual holes do 

 occur in the membrane of the parenchymatous cells of most herbaceous and 

 succulent plants when they are old, for then the original or primitive mem- 

 brane which closed the transparent spot is absorbed ; this may easily be 

 seen in autumn, when succulent plants are killed by the first frost. Even in 

 the membrane of the parenchymatous cells of the Tradescantia I found 

 holes at this season, although in summer not even transparent spots were 

 to be seen. It is just the same with the large transparent spots on the 

 sides of the parenchymatous cells in the leaves and leaf-stalks of the Cycadecs; 

 in the ferns, the palms, in short, in every case, where at an earlier period 

 there are only transparent spots, these pores may make their appearance, 

 but one is soon convinced that in the interior of such porous cells no circu- 

 lation of the sap and no new production can take place. These formations 

 might therefore reassume their old name of pores, if, indeed, it were any 

 improvement. 



