UUCURB1TACE.E 597 



occurring in this group, or indeed in the whole Order, is that 

 of Acanthosicyos horrida which bears organs that are morpho- 

 logically leaves but are functionally useless as such. The 

 first leaf is small, linear, entire, concave and carinate. From 

 the second to the fifth inclusive they are gradually smaller 

 and subtended by a pair of spines, but otherwise similar 

 to the first. This, the only species of the genus, is a native 

 of the dry sandy deserts of Angola and Dammara Land; 

 and the reduction in size or almost entire abortion of the 

 leaves is the expression of the adaptation of the plant to its 

 environment. 



Besides the above types a few others may be mentioned 

 which differ so much from the leading ones and from one 

 another as far as the cotyledons are concerned that they may 

 be regarded at any rate for the present as exceptional. The 

 cotyledons of Bryonia dioica (fig. 391) are suborbicular, entire, 

 suddenly euneate at the base, with slender, considerably 

 elongated petioles. They are small compared with those of 

 B. laciniosa, and altogether different in shape. The elongation 

 of the petioles would appear to be necessary owing to the 

 hypocotyl being almost suppressed or scarcely rising above 

 ground. Thladiantha dubia (fig. 385) has a relatively short 

 hypocotyl, suborbicular, broadly and shallowly emarginate 

 cotyledons, with comparatively long petioles, all of which are 

 rather thickly strewn with stiffish hairs. 



An extraordinary instance of another kind occurs in 

 Megarrhiza californica, the hypocotyl of which is short, 

 subterranean, and together with the upper portion of the 

 root becomes developed into an enormously thickened and 

 fleshy tuberous structure. The cotyledons are large, fleshy, 

 plano-convex, and never leave the large, oblong, tumid seed 

 although they are carried two or three inches above ground 

 by the petioles. The latter appear further to be connate into 

 a cylindrical tubular body tapering upwards, but they are 

 really separable. The plumule proceeds from a slit between 

 the petioles close to the base of the latter at the level of the 

 ground or below it. The first three or four leaves are reduced 

 to small scales. All this growth preliminary to the develop- 

 ment of normally expanded leaves is produced at the expense 



