40 



HISTORY OF THE VEGETABLE KINGDOM. 



of compound leaves, they ai'e then called decom- 

 l>onnd, or doMj/ compound, fig. /. The simply 



compound leaves have two principal modifica- 

 tions, depending on the position of the leaflets 

 that compose them. Thus, sometimes all the 

 leaflets proceed from the very top of the common 

 petiole, as in the horse chestnut, and trefoil; some- 

 times again they spring from the sides of the 

 common petiole, as in the ash, bladder senna, 

 &c. The name of digitate has been given to the 

 former, and pinnate to those of the latter. Di- 

 gitate leaves, then, are those which spring in a 

 divergent manner from the top of the common 

 petiole, like the fingers of the hand spread out. 

 They are unifoKate, as iu the leaf of the orange 

 tree ; trifoliate, as in wood-sorrel, qwinquefoliate, 

 as in potentilla, &c. 



Pinnate, leaves arranged on each side of a 

 common petiole, like the barbs of a feather in 

 the shaft. 



Decompound, where the common petiole is 

 subdivided into other petioles, being compound 

 leaves, as in mimosa, fig. g.* 



Supradecompound, are those in which the 

 secondary petioles ai'e divided into tertiary 

 petioles, bearing leaflets, fig. h. 



We now proceed to explain the functions of 

 these essential organs of plants. 



Leaves, as before observed, are formed of three 

 principal parts, namely, abundlc of vessels coming 

 from the stem ; cellular substance, which is a 

 prolongation of the herbaceous envelope of the 

 bark ; and, lastly, an outer skin or epidermis, by 

 which they are covered in their whole extent. 



The bundle of vessels constituting the petiole. 



• Particular explanations of all tlio terms will be 

 fnnnd in tlie glossary at the end of tliia work. 



when this organ is present, consists of trachesp, 

 false tracheae, and porous vessels. They are ex ■ 

 temally enveloped by a layer of the herbaceous 

 substance, which is prolonged over them when 

 they come ofl^ from the stem. Bv their expan- 

 sion and successive ramifications, they form the 

 network of the leaf. The meshes or empty 

 spaces which they leave between them are filled 

 with cellular tissue coming from the bark. This 

 tissue is sometimes wanting, as when the leaf 

 consists of vascular network alone, presenting 

 the appearance of a kind of lattice-work or 

 lace. 



The epidermis which covers the surface 

 of the leaf is generally thin and very porous, 

 especially on the under side. The two layers 

 of this organ, seen upon the upper and under 

 surfaces of the leaf, cover the part which is 

 formed by the vascular fibres and cellular tissue. 

 This organ is very thin, in flat and membranous 

 leaves; but in such as are thick and fleshy, as 

 those of succulent plants, it is greatly developed, 

 and gives the leaf its form. 



The stomata or pores which are observed on 

 leaves, are, according to some authors, nothing 

 but the upper orifices of the sap- vessels ; while 

 others maintain that they transmit air. 



The leaves and roots are the principal organs 

 of absorption and nutrition in vegetables. The 

 former absorb from the atmosphere the substances 

 which are subservient to growth, and hence they 

 have been called aerial roots. They are also 

 subservient to other purposes of gi'eat importance 

 in the economy of plants. They transpire and 

 exliale the fluids which have become useless to 

 vegetation, and it is by their agency that the 

 sap is freed of the watery juices which it con- 

 tains, and acquires all its nutritive qualities. 



It is chiefly by the pores situated on the lower 

 surface of the leaves of woody plants that the 

 watery vapours and gases diff^used in the atmos- 

 phere are absorbed ; and hence this surface is 

 softer, and less smooth than the upper, and is 

 generally covered by a light down which is 

 favourable to this absorption ; while the upper 

 surface, on the contrary, is smoother, generally 

 glabrous, and throws off the fluids which are 

 useless for the nutrition of the plant. This ex- 

 cretion is named transpiration in vegetables. 



The leaves of herbaceous plants, being nearer 

 the ground, and immersed in a constantly humid 

 atmosphere, absorb equally I)y both surfaces. 

 The knowledge of this fact we owe to the cele- 

 brated Bonnet. The most complete sot of ex- 

 periments, upon the absorbent power of the 

 leaves, is detailed by that naturalist. Satisfied 

 that leaves are furnished with absorbent organs 

 for the purpose of taking in moisture, his object 

 was that of ascertaining whether the absorbent 

 power of both surfaces was alike. With this 

 view he filled several vessels with water, on the 



