xix] DELAYED GERMINATION 243 



schizocarp of Callitriche divides, the same function is performed 

 by the pericarp, which is thin, but tough and elastic. The well- 

 protected seeds of hydrophytes can in many cases withstand 

 inclusion for a considerable length of time in ice or frozen mud. 

 The fruits of Sagittaria sagittijolia, Alisma Plantago and Myrio- 

 phyllum spicatum^ and the seeds of Castalia alba and Nymphaea 

 lutea can tolerate freezing for a week or two, or, in some cases, 

 much longer 1 . 



With the particularly effective protection of the embryo in 

 hydrophytes, their characteristic habit of delayed germination 

 is probably to be associated. The sprouting of the seed may in 

 some cases be deferred until the third, fourth, or fifth year 2 , 

 the embryo remaining uninjured by this prolonged period of 

 dormancy. 



Several investigators have studied the subject of delayed 

 germination, and the rather curious fact has emerged that this 

 delay only occurs if the seeds are continuously immersed in 

 water; if they are subjected to a period of drying, they germinate 

 promptly. It has been noted, for example, that the seeds of 

 Mayacafluviatilis, a Brazilian water plant, which were dried for 

 six weeks after gathering, germinated at once, while seeds 

 harvested at the same time, but put immediately into water, 

 showed no sign of sprouting at the end of three months 3 . The 

 seeds of some water plants can tolerate drying for a very long 

 period, e.g. thirty months in the case of Limnanthemum nymph- 

 oides*. The result of experimental work on the subject seems to 

 be to show that the delayed germination of undried seeds is due 

 to the mechanical pressure exerted by the seed coats 5 ; if these 

 are artificially ruptured, the development of the embryo presents 

 no further difficulties. It has been found, for instance, that in 



1 Guppy, H. B. (1893) and (1897). 



2 Guppy, H. B. (1897) ; on delayed germination in Potamogetons 

 see pp. 71, 72, and in Nymphaea, p. 36. 



3 Ludwig, F. (1886). " 4 Guppy, H. B. (1897). 



5 Sauvageau, C. (1894), Crocker, W. (1907), and Crocker, W. and 

 Davis, W.E. (19 14). For a somewhatdifferent viewsee Fischer, A. (1907). 



1 6 2 



