666 



ECOLOGY 



great depth of 6.5 mm. in Cereus giganteus. Synthetically comparable to 

 cacti are many switch plants, with numerous leafless switchlike stems 

 (as in Ephedra, fig. 975). Some switch plants, as Spartium and Cy- 

 tisus, have small leaves, which have been shown to equal or surpass the 

 stems in synthetic activity. 



Asparagus and Casuarina have numerous slender branches, which give the aspect 

 of delicate foliage. Equisetum (figs. 1054, 1055) is a characteristic leafless herb. 

 In Muehlenbeckia the stems, though vertical, are much flattened, and in Myrsiphyl- 

 lum, Ruscus, and Phyllocladus they quite resemble leaves and are called phylloclades. 

 Many such plants have prominent leaves in the seedling stages, suggesting, accord- 

 ing to the recapitulation theory, that various leafless xerophytes may have been 

 derived from a mesophytic leafy ancestry; the phylloclade forms have been thought 

 to represent a subsequent return to more mesophytic structures. 



Not all leafless stems are xerophytic. Such representative swamp 

 plants as the rushes (Juncus, Scirpus, Eleocharis) often are essentially 

 leafless, the synthesis of foods being here a stem function, as in the 

 cacti (fig. 976). In their high cutinization and prominent palisades 



such plants resemble xerophytes, 

 but they are quite unlike them in 

 their abundance of air spaces and 

 in their high transpiration. It has 

 been suggested that their ex- 

 posure to intense light, reflected 

 as well as direct, makes vertical- 

 ity almost as advantageous as 

 in xerophytic habitats. It is 

 much more likely that in rushes, 

 as in swamp grasses and in flags, 

 vertically is advantageous, be- 

 cause it permits a maximum 

 display to light where grow th is 

 dense. Whatever may be the 

 causes or advantages, it ceri ainly 

 is striking that leafless stems 

 with a relative maximum of stem 

 synthesis occur in such opposite 

 habitats as deserts and swamps, 

 and that the vertical habit which 



FIG. 977. The upper part of a gametophytic 

 stem of Sphagnum, above which is displayed a 

 terminal cluiterof sporophytes with their spore- 

 bearing organs; note the descending gameto- 

 phytic branches, which facilitate the ascent of 

 water by capillarity. From COULTER (Part I). 



means minimum light exposure 



