10 



HISTORY OF THE VEGETABLE KINGDOM. 



vegetables. WMle the soft portion, composed 

 of cellular tissue, is called the parenehyma, con- 

 stituting the pulp of tiiiits, interstices of leaves, 

 &c. This tei-m is used in opposition to fibre, 

 every part which is not fibrous being composed 

 of parenchyma. These two tissues, combined 

 in various ways, make up the different organs of 

 plants ; the vascular tissues consisting, as we 

 have seen, of, 1st, The sap vessels, or lymphatics, 

 in which the sap is circulated. 2d, The simple 

 vessels, containing the peculiar or proper juices 

 of the plant. 3d, The air vessels, in which we 

 never find any thing but elastic gases. But the 

 different writers on vegetable physiology are 

 far from agreeing on the class to which the dif- 

 ferent species of vessels belong. Thus, many of 

 the older, as well as the more recent writers in 

 botany, are of opinion, as already stated, that 

 the spiral vessels contain gaseous fluids alone, 

 while Mirbel has denied the existence of air 

 vessels at all, and maintains, that all the tubular 

 vessels of vegetables are destined solely for the 

 circulation of sap. Professor Amici, on the 

 other hand, affirms positively, that he has ascer- 

 tained by observation, that the spirals, the false 

 spirals, the porous vessels, and in general all the 

 tubulai' and cellular organs of vegetables which 

 have visible holes or slits, never contain any 

 thing but air. When the diameter of these 

 tubes is large enough, this observation can easily 

 be verified by cutting the tubes across, they are 

 then observed to be always empty. If the di- 

 vision be made under water, each of them is 

 seen to present a small air bubble at its orifice. 

 The openings or pores with which the porous 

 vessels are perforated, are very frequently organ- 

 ized hke the pores of the epidermis or outer skin, 

 that is, they present at their circumference a 

 circular swelUng, or border. This remark made 

 by Mirbel, has been confirmed by Amici. From 

 this resemblance the latter draws a conclusion 

 which is favourable to his opinion, respecting 

 the nature of the fluid contained in these vessels. 

 In fact, the great pores of the epidermis never 

 give passage to any other than elastic fluids. 

 The air contained in the porous vessels does not 

 communicate with the external air. Amici 

 thinks it is produced in the interior of the vege- 

 table tissue ; but its nature is not as yet perfectly 

 known. In woody vegetables, where the air 

 vessels ultimately disappear, their place is occu- 

 pied by the medullary rays, which perform the 

 same ftinctions. These are, in fact, composed of 

 small tubes placed horizontally, or of porous 

 cells elongated in a transverse direction, which 

 seem to serve as a medium of communication 

 between the inner parts of the vegetable and the 

 outer. These tubesorceUsnevercontain anything 

 but air. From the descriptions given then, it 

 will be observed that there me two principal 

 means of communication between the different 



parts of the vegetable tissue. In the air cells, 

 or tubes, the communication is preserved by 

 means of pores or minute slits. These pores 

 are altogether wanting in the cellular tissue, 

 properly so called ; and in the vessels called sim- 

 ple tubes or proper sap vessels. In that part of 

 the vegetable tissue, the communication takes 

 place either by a kind of imbibition, or by the 

 intervening spaces which the globules that com- 

 pose the layers of that tissue leave between 

 them. 



Pores. These are small and minute openings 

 of various shapes and dimensions, adapted for 

 the absorption, transmission, or exhalation of 

 fluids ; and have, by some, been classed under 

 perceptible and imperceptible pores. The per- 

 ceptible pores are either external or internal, and 

 are the apertures described by Hedwig as dis- 

 coverable in the net-work of the epidermis, or 

 by jNIirbel as perforating the membranes com- 

 posing the cells and tubes, and forming a com- 

 munication between them. The stomata or leaf 

 pores, wUl be more particularly described ivhen 

 treating of the structure of leaves. They are 

 found in considerable numbers in the softer par- 

 enchematous structure of the leaf, and rarely or 

 never on the stems or fibres : on the under side 

 of the leaf of nymphea or water lily, or on the 

 lettuce or common cabbage leaf, they may be 

 distinctly seen. On them they are, however, 

 discoverable on both surfaces of the leaf, exhib- 

 iting an oval aperture more or less dilated, to- 

 gether with communicating ducts. On the upper 

 surface they £ire much fewer and smaller than 

 on the under ; and in the leaves of trees, they 

 are fewer and smaller on both surfaces, than in 

 the leaves of herbs. They are generally oval ; 

 in the nymphea they are round and not readily 

 detected, the epidermis of this plant being very 

 difficult of detachment. The internal pores, or 

 apertures, forming the medium of communica- 

 tion between the different cells and tubes, have 

 been already described. In some plants, they 

 _are but few and scattered, and in others, they 

 are numerous and arranged in regular rows, 

 which extend always in a transverse, never in a 

 longitudinal direction, being destined, probably, 

 for the lateral transmission of the sap. The 

 imperceptible pores are not distinguishable even 

 by a powerful microscope ; but they are pre- 

 sumed to exist by the evidence of experiment. 

 In the fine pellicle of pulpy fruits, though 

 exliibiting evidently traces of organization, no 

 pores have as yet been discovered. But we must 

 not on that account conclude that it is altogether 

 without pores ; on the contrary, we must assume 

 their existence, because it is very well known 

 that the fruits in question both absorb and tran- 

 spire moisture ; and if so, there must of neces- 

 sity exist apertures for the passage of moisture. 

 The diameter of such, however, must be extremely 



