678 



ECOLOGY 



advance upstream, and even may migrate from one river or pond to 

 another; wind and fish may be agents of dispersal upstream, and it is 

 the current belief that the feet of wading birds are important agents 

 in carrying plant fragments from one pond to another. Clearly the 

 culmination of vegetative reproduction among seed plants is to be seen 

 in fragmenting hydrophytes, where stem detachability facilitates dis- 

 persal as much as does seed production. 



Winter buds. In many aquatics there develop special winter buds or hibernacula, 

 which readily become detached and drop to the bottom of the pond because they are 

 heavier than water. In the duckweeds this heaviness is due to the relative lack of 



FIG. 997. Plants of one of the smallest 

 of the duckweeds (Wolffia), whose body 

 is reduced to a thallus (/); note that 

 vegetative reproduction occurs through 

 the development of a bud (b\ which be- 

 comes detached at maturity, as at m; 

 note also that a part of the thallus is 

 below and a part above the water, the 

 former having to do chiefly with the ab- 

 sorption of water and salts and the latter 

 with food -making and transpiration ; con- 

 siderably magnified. 



FIGS. 998, 999. Winter buds (hiber- 

 nacula) of the bladderwort (Uiricularia) : 

 998, a portion of a shoot of the land form 

 of Utricularia intermedia, showing a hori- 

 zontal axis, terminating in a winter bud 

 (h) ; note also that the progeotropic earth 

 shoots have terminal winter buds (h') and 

 conspicuous bladders (6) ; 999, a winter 

 bud of Utricularia vulgaris. From 

 GLUCK. 



air spaces, while in Utricularia, it is due to the compact growth of either the ter- 

 minal or the lateral buds, the stem ceasing to elongate and the leaves being very 

 closely imbricated; the whole forms a somewhat globular structure (figs. 998, 999). 

 In spring most hibernacula develop roots, and the stem elongates into an ordinary 

 vegetative shoot; however, in the duckweeds and in Utricularia, soil roots do not 

 appear, and the developing plants soon become lighter than water and rise to the 

 surface. In Utricularia, winter buds may be induced experimentally at any season. 

 The chief advantage of hibernacula is the preservation of the species over inclement 

 seasons; however, they represent new plants that have arisen vegetatively from the 

 parent individual and thus are reproductive structures. 



3. CONDUCTIVE TISSUES 



General remarks. Water and solutes may move from any cell to any 

 adjoining cell, if the walls are permeable to the diffusing substances. In 

 most small plants special conductive tissues are absent ; also they are 



