SUCCULENT PLANTS, AND CLADODES. 



55 



African, as Euphorbia c'anariemts, globosa, Caput medusa, &c. The assimilating 

 tissue containing chlorophyll is, in these leafless succulents, extended in the form of 

 a very thin layer beneath the epidermis of the shoot-axis. 



We find weak, thin shoot-axes, with very succulent leaves, in the family 

 of the Crassulaceae, and in the genus Mesembryanihemum ; among the Mono- 

 cotyledons especially in the genus Aloe, and here and there also in other plants. 

 The leaves in these cases entirely give up their usual expansion as thin laminae ; 

 they become thick, tuber-like, or prismatic, and so forth. The parenchyma con- 

 taining chlorophyll forms also in them only a thin layer extending beneath the 

 epidermis, while the main mass of such leaves consists of colourless, succulent, slimy 

 tissue. 



In contrast to the Succulents, there are also plants the shoot-axes of which 

 remain thin, and possess abundant chlorophyll beneath the epidermis, while the 

 leaves are all arrested as small scales; so that here also, as in the Cactus-like 

 succulent plants, the shoot-axes themselves undertake the function of assimilation. 

 Such plants are found in various families of Phanero- 

 gams ; among the Papilionaceae, e. g. Spar Hum junceum, 

 and among the Scrophularinse, Russelia juncea, &c» The 

 whole Cryptogamic class Equisetacese (Horsetails) also 

 belongs here, as well as the genus Psilotum included 

 in the Lycopodiaceae. 



Especially instructive, with reference to the relations 

 between axis and leaf, are the plants which produce so- 

 called Cladodes. These are shoots, the leaves of which 

 are arrested as very small scales, whQe the shoot-axes 

 themselves assume the flat form of ordinary leaves, to 

 such an extent that an unpractised observer would un- 

 doubtedly take shoot-axes of this kind for ordinary 

 foliage leaves. Among the Monocotyledons the genus 

 Ruscus from the family of the Asparaginese, among 

 the Coniferse the genus Phyllocladus from the family 

 of the Taxineae, and among the Dicotyledons the genus 

 Phyllanihus from the family Euphorbiaceae, require special 

 mention. 



In such cases, it is generally only lateral shoots of the main axis which assume 

 the form of thin foliage leaves; it may also occur, however, that all the shoots, even 

 if not exactly leaf-shaped, are flattened, broad, and abound in chlorophyll, and so 

 replace the arrested foliage leaves, as in Muhlenbeckia platyclada (Polygonacese), and 

 in Carmichaelia (Papilionaceae). 



In the cases of metamorphosed shoots mentioned hitherto, it is essentially 

 a question of the distribution of the assimilating tissue containing chlorophyll, 

 either in the leaves or in the shoot-axis itself. The climbing shoots, on the other 

 hand, belong to quite another category of metamorphoses, in which the relation 

 of the foliage leaves to the shoot remains essentially undisturbed ; where, how- 

 ever, arrangements exist by means of which the feeble, thin, shoot-axes, which 

 are not able to bear the weight of the leaves, flowers, and fmit, are enabled to 



FIG. 4g.^Xuscus acuieatur (after 

 Duchartre). a main shoot of the usual 

 form ; eld the leaf-like lateral shoots 

 (cladodia) on which the flower shoots 

 [Jl) are situated. 



