REPORT ON THE PHILOSOPHY OF BOTANY. 29 



pores are nothing more than grains of amylaceous matter stick- 

 ing to the sides of the sacs ; for he found that by immersing the 

 latter in hot nitric acid, the supposed pores became opake, and 

 by afterwards moistening them with a weak solution of caustic 

 potash, they recovered their transparency : we also find that 

 the supposed pores are readily detached from the sides of the 

 sacs to which they adhere ; and I think it may be added, that 

 our microscopes are now alone sufficient to show what they are. 



The question as to the perceptible porosity of vegetalile tis- 

 sue may therefore be considered, I think, disposed of as a 

 general fact; for the objection that Dr. Mohl has taken to this 

 explanation *, — namely, that in a transverse section we ought to 

 find such grains projecting from the sides of the cells like little 

 eminences, — cannot surely be entitled to much weight, if we op- 

 pose to this negative observation the positive evidence already 

 mentioned, and especially if we consider that it is next to im- 

 possible for the keenest knife to make a section of such delicate 

 parts without carrying away such particles upon its edge. There 

 are, nevertheless, cases in which the point is still open to in- 

 vestigation. 



Thus Mirbel, in his second memoir on the Marchantia f , 

 positively declares that the curious cells which line the anther 

 of the common gourd, are continuous membi'anes till just be- 

 fore the expansion of the flower, when they very suddenly en- 

 large, and their sides divide into the narrow ribands or threads 

 which give their name to what we call fibrous cells. In this, 

 and the multitudes of similar cases with which Purkinje has 

 made us acquainted, there can be no doubt that the sides of 

 the cells consist ultimately of nothing but openwork; but still 

 it seems certain that during the principal part of their existence 

 they were completely closed up. 



It is also probable that in other cases the sides of the cells 

 or vessels ultimately give way and slit; but this rending seems 

 to be a phaenomenon attendant upon the cessation of the ordi- 

 nary functions of tissue, and independent of their original con- 

 struction. 



In coniferous plants the wood is in a great measure com- 

 posed of closed tubes, tapering to each end, the sides of which 

 are marked Avith circles, containing a smaller circle in their cen- 

 tre. These circles have long been considered undoubted pores^ 

 and it does not appear possible to prove them otherwise by any 

 of the tests already mentioned. 



• Ueler die Poren des Pflanzen-Zellgewehes, p, 11. Tubingen, 1828. 

 t Archives de Botanique, vol. i. 



