INTRODUCTION. XXI 



and most Iridese, have a rhizoma in one form or other, and bulbs are 

 a usual character of Asphodeleae and Amaryllideaj ; in the former, 

 however, the bulb is often represented by a rhizoma, or cormus, 

 as in Brodiaia, Leucocoryne, and their allies, or by those succulent 

 fibres called fasciculate roots, as in Asphodelus itself; and in the 

 latter the bulb is sometimes entirely absorbed by succulent perennial 

 leaves, as in Clivia. 



External variations in the figure of the stem are sometimes 

 available as distinctions of orders. Thus, a twining stem is almost 

 without exception in Menispermese, a square stem is universal in 

 Labiatae, and an angular one in Stellatae ; but more frequently its 

 figure affords no indication whatever of affinities. — Texture of the 

 stem is of scarcely more value. Cacteas, it is true, have always the 

 cellular tissue in excess, and derive by that circumstance one of 

 their great distinctions from Grossulacese ; but even in Cactese 

 the Pereskias are scarcely more succulent than other plants ; and 

 Euphorbiaceai and Asclepiadeae exhibit instances both of the most 

 decided state of anamorphosis, and of the normal condition of 

 stems in general. — In the internal arrangement of the layers of 

 Exogenous stems, I am not aware of any character which distin- 

 guishes orders besides those to which I have already adverted ; 

 except in Calycantheaj, which are distinctly known by the presence 

 of four incomplete centres of vegetation surrounding the principal 

 one, and so forming four angles which are visible externally. (See 

 Mirbel's figure, in the Annales des Sciences, vol. xiv. p. 367.) But 

 as I have before observed, very little is really known upon this 

 subject. 



The Leaves are subject to modifications not less important in 

 determining the mutual relations of plants, than the functions which 

 they perform in the vegetable economy. Their characters depend 

 upon their relative position, their degree of division, their venation, 

 and the presence or absence of pellucid dots within their sub- 

 stance. — All Cinchonacese (Rubiacese) have opposite entire leaves ; 

 in Labiatae, Apocyneae, Gentianeae, Monimieae, and many others, 

 they are also uniformly opposite ; but in the genus Fuchsia, in which 

 they are usually opposite, species exist in which they are not only 

 alternate, but both the one and the other on the same plant,- and 

 alternate-leaved species exist in Compositae, Scrophularineae, and Mal- 

 pighiaceae, orders the leaves of which are generally opposite. In 

 Cupuliferae, Umbelliferae, Ternstromiaceae, Hamamelideae, and Ur- 

 ticeoe, they are uniformly alternate ; but in Combretacese and Legu- 

 minosae, orders usually having alternate leaves, they are occasionally 

 opposite ; and Halorageae, Ericineae, and Ficoideae, are orders in 

 which the genera have their leaves arransred in no certain manner. 



