286 The Rev. P. Keith on the Internal Structure qf Plants. 



Mirbel than ever Kieser was. It is to the eclaircissemens of 

 Dutrochet that we are indebted for any new light that has 

 been lately thrown on the structure of the sap- vessels. Still 

 we cannot do without Mirbel. We must have him, if it were 

 but for the sake of refuting him, as the defenders of our faith 

 must have the books of the infidels. 



The vessels of plants are divided by Mirbel, into sap-ves- 

 sels, and proper vessels. Of the sap-vessels he enumerates 

 five species, — porous tubes, spiral tubes, false spiral tubes, 

 mixed tubes, and cellular tubes — les vaisseaux en chapelet. Of 

 the proper vessels he has but one species, which he calls sim- 

 ple tubes. 



The first species of tubes containing sap are the porous 

 tubes. They are described by Mirbel as having compara- 

 tively a considerable width of diameter, and as being pierced 

 with multitudes of minute holes or pores distributed in parallel 

 and transverse rows. They abound in woody plants. They 

 are not continuous from the base to the summit; but they 

 unite, separate, and unite again, and finally disappear by 

 changing into cellular tissue. The doctrine of porous tubes was 

 attacked by the German phytologists with as much hostility as 

 was that of porous cells. But it met also with some strenuous 

 defenders ; and even the antiporists themselves were obliged 

 to admit that the vessels in question are studded with lumin- 

 ous points, or with little transparent vesicles, arranged as 

 the pores are said to be ; so that at all events Mirbel's account 

 is not a fiction, but a mistaking of the character of certain 

 given appearances. Thus the matter might have been said 

 to be left in doubt. But as Dutrochet has searched for the 

 pores of the tubes, as he searched for the pores of the cells, 

 without being able to find them ; and has, at the same time, 

 adduced strong reasons for supposing them to be merely small 

 and minute molecules imbedded in the substance of the mem- 

 brane that forms the vessels*, we regard the doctrine of porous 

 tubes as being no longer tenable. 



The second species of tubes conducting sap is the spiral 

 tubes — a name not imposed, but adopted, by Mirbel, as being 

 long in use. They are fine, transparent, and thread-like sub- 

 stances, occasionally interspersed with the other tubes of the 

 plant, but readily distinguished from them by their being 

 twisted in the form of a corkscrew from right to left as in the 

 stem of Spearmint ; or from left to right as in the stem of 

 Fuller's Teasel. Grew and Malpighi, who first discovered 

 and described them, represented them as resembling in their 



* Recherchcs Anatomiques, p. 27- 



