184? The Rev. P. Keith on the Internal Structure of Plants. 



of the parenchyma, and compares it to the bubbles formed 

 upon the surface of liquor in a state of fermentation. Du 

 Hamel represents it as consisting of a sort of network of fi- 

 bres, crossing in all directions, and interspersed with small 

 and granular, or bladder-like substances occupying the inter- 

 stices. 



Such are the descriptions of the earlier vegetable anatomists, 

 in one or other of which the general appearance of the pulp 

 will be found to be pretty fairly represented under whatever 

 aspect the botanical student may happen to meet with it. But 

 later anatomists have been more minute. Mirbel describes 

 it as being composed of clusters of small and hexagonal cells, 

 containing a juice. This was an important step in advance, 

 yet he obscures his description in the introduction of a useless 

 distinction, by which he divides his cellular tissue into a pa- 

 renchyma and a herbaceous tissue, the former containing a 

 coloured, the latter a colourless, juice. But an apparatus of 

 united cells, containing a fluid whether colourless or coloured, 

 is all that is necessary to form a true pulp. The cells he does 

 not regard as being distinct and individual organs, such as 

 might be insulated and shown separately, but merely as be- 

 ing formed of a fine and delicate membrane so folded or 

 doubled up as to leave the partitions single and common to 

 two cells. This doctrine Dutrochet has shown to be ground- 

 less, and if we repeat his experiment we shall come to the 

 same conclusion. Take a portion of pulp and put it into a 

 phial filled with nitric acid. Plunge the phial thus rilled into 

 boiling water, and the cells will soon become easily detach- 

 able, or will soon begin to separate and to present themselves 

 entire in their hexagonal form. Hence, where the walls touch, 

 the membranes must be double # . 



Further, Mirbel regards the partitions of the cells as being 

 perforated by minute holes, or pores, for the transmission of 

 sap or other juices of the plant. Yet his doctrine of internal 

 and visible pores has not been generally adopted by other 

 vegetable anatomists. It was attacked and denied, rather 

 rudely, by the German doctors Sprengel and Treviranus, 

 and again affirmed and reasserted by Mirbel in his Defense 

 de ma Theorie, as well as by Link, Hedwig, and Rudol- 

 phi. But as the pores in question have been still more re- 

 cently and more laboriously searched for by M. Dutrochet 

 without success, I suppose we must be content to regard the 

 doctrine as unfounded. Dutrochet says the pores are merely 

 small globules imbedded in the walls of the cells f. It should 



* Rccherches A?iatomiqucs, torn. xi. f Ibid. torn. xiii. p. 40. 



