8 THE GENUS PHOEADENDEON 



STEM. Though it frequently happens that only one of the two 

 opposed buds at a node develops into a branch, so that a pseudodicho- 

 tomous forking may appear, the greater number of species, including 

 all of those in our own flora, are monopodial or percurrent in their 

 growth ; but a comparison of P. flavens and P. racemosum, for example, 

 among the West Indian species, shows that in the former the percurrent 

 growth is very constant, while in the latter the main axis is so rarely 

 continued that forking, or, through accessory development, fasciculation 

 of the stems is all but the universal rule. In P. cymosum and a group 

 of related species the suppression of the main vegetative stem is further 

 accentuated through its replacement by a flowering spike, so that the 

 seeming dichotomy of P. racemosum is here replaced by a cymose forking. 



While all of our own species have a terete or nearly terete stem, 

 squarish in some of the mountain forms, such a species as P. vernicosum 

 presents the phenomenon of its compression into an elliptical cross sec- 

 tion below the nodes ; in P. carneum, etc. it is sharply 2- keeled : in P. 

 peruvianum, etc., it is convexly sword-shaped, and it becomes 2- winged 

 in P. dipterum or even very thin and broad in P. platycaulon. P. rubrum 

 and many other species have a comparable sword-like compression 

 accompanied by a rhombic keeling of the broad surface, with extremes 

 from little to marked widening reaching its culmination in the very 

 broadly winged stems of the Mexican species which Hooker mistook for 

 Viscum falcatum (PI. 62, 63). P. trinervium and a number of other 

 tropical species have this rhombic keeling amplified into a sharply and 

 nearly equally 4- angled character, which in P. tetrapterum and a few 

 others develops into a strong and often undulate winging. As a rule 

 these stem peculiarities are most evident on the uppermost internodes 

 of a branch ; sometimes they disappear entirely as the stem ages, or are 

 represented by a faint lining on otherwise nearly or quite terete older 

 internodes: in one species, P. paradoxum, terete-based and ancipital 

 internodes regularly alternate in the branches. 



LEAP. If, as is the case, leaf -form in this genus varies in the same 

 species or even on the same branch so greatly as to prevent its use with 

 precision for the differentiation of closely related forms, and though 

 identical shapes may be presented by the leaves of species not at all 

 related, the foliage of a given species comes with familiarity to present 

 a collective effect that is characteristic so far as it goes. Knowledge of 

 the species when growing is certain to reveal very marked differences 

 in texture, veining, and direction of the leaves which are lost or uncertain 

 in the herbarium; but even in dried specimens many foliage charac- 

 ters may be picked out. In P. Eggersii and a relatively small number 

 of other species, distinct clean cut petioles are found, while in 



