STEMS 667 



and maximum protection for the desert individual means maximum light 

 exposure for the mass of vegetation in the swamp. 



The display of reproductive organs by stems. — In the lower groups, 

 stems seem to be associated with the display of reproductive rather than 

 synthetic organs, the obvious advantage being the facilitation of spore 

 dispersal by wind and by other agents. In the fungi, where carbohy- 

 drate synthesis does not take place, there are prominent stalks or stipes 

 tipped by the spore-bearing organs (as in various molds and toadstools, 

 figs. 1078, 1 122, 197). While the leafy shoots and thalli of liver- 

 worts generally are closely appressed to the substratum, most species 

 have stalked spore-bearing organs that facihtate dispersal (fig. 235). 

 In the mosses the organ {seta) that bears the capsule with its asexual 

 spores is elevated above the rest of the plant (fig. 977). In the pterido- 

 phytes the sporangia may be borne on ordinary leaves or on special 

 leaves or stems (as in Osmunda, Equiseium, Lycopodium, fig. 266). In 

 the seed plants the display of reproductive organs may be still more 

 advantageous than in the lower groups, by reason of the important 

 part played by insects in polhnation and by birds in seed dispersal. 

 Even rosette plants (as Taraxacum and Agave, figs. 869, 922) usually 

 have tall stalks on which the reproductive organs are borne. In some 

 aquatics the flowers are erected above the water on special stems. 

 Colorless seed plants (as Monotropa) resemble fungi in having stems 

 that display only reproductive organs. 



2. STEMS AS REPRODUCTIVE ORGANS 



General remarks. — Plants spread in various ways, involving ordinary 

 vegetative organs or specialized organs, such as seeds and spores. 

 While the latter are the more effective dispersing agents over great dis- 

 tances, vegetative reproduction is much more effective as a means of 

 occupying the ground in the immediate vicinity of the parent plant. 

 Among such organs of vegetative reproduction stems take the foremost 

 place. 



Rhizomes or rootstocks. — General features. — Rhizomes or root- 

 stocks are horizontally elongated underground stems, which occasionally 

 compose the entire stem system of the plant (as in various violets and in 

 most ferns), but which more commonly give rise to erect annual stems 

 that bear foliage leaves and flowers (fig. 978). In the latter case the 

 rhizome bears only scale leaves (figs. 979, 980), in whose axils erect 



