448 THE GERMINATION OF HIPPURIS VULGARIS. 
a loose substratum as mud, which is easily silted, any mechanism which 
tends to keep the ehlorophyll-bearing organs above the surface must be of 
some importance. In ordinary land plants the radicle is, of course, firmly 
held in position by the soil particles. 
Mrs. Arber, in her book on * Water Plants,” and others have stated that 
the fruits of //ippuris winter on the mud and germinate in the following 
spring. To test this statement ripe fruits of the current season were obtained 
through the kindness of Mr, Gilbert Carter of the Cambridge Botanic 
Garden, and immediately sown under conditions similar to those mentioned 
above. At the time of writing (end of November) no germinations had 
occurred *. This helps to confirm the above views, since it can hardly be 
expected that germination would commence in mid-winter. Fauth (loe. cit.) 
states that he has seen germination in progress in April, and also næntions 
that fruits dried over the winter and sown in March germinated in June. 
It will be remembered that the seedlings here described were from fruits 
which had been naturally dried for some time. 
Fischer (in Ber. Deut. Bot. Ges. xxv. p. 108) states that the fruits of many 
aquatics, including //ippuris, can be kept for years, without germinating, in 
pure (distilled) water. If, however, fermentation occurs, the action of the 
H + and OH — ions stimulates the dormant protoplasm to growth. On the 
other hand, Crocker (in Bot. Gaz. xliv. p. 376) considers that abundance of 
oxygen and water are chiefly necessary. 
Hippuris fruits are covered when ripe by a complete outer covering of 
thin-walled tissue, and it appears that germination does not occur until this 
layer, which seals the enclosed embryo, is removed. Its removal may be 
under water by the ordinary processes of decay or by mechanical means 
within the gizzards of birds. In either case germination seems to follow 
readily. Probably the removal of the outer tissues allows the external 
factors of stimulation, whatever they may be, to act upon the embryo. In 
the presence of the outer layers of the fruit the actual movements of germi- 
nation, such as the pushing out of the stopper, must be greatly impeded, if 
not actually prevented. If this explanation of the delay in germination is 
acceptable, it is correlated with the fact that the fruits of //ippuris ripen 
late in the season, so that immediate germination would leave the young 
seedlings to pass the winter in a very critical stage of development. 
In conclusion, there seems no reason for believing that passage through 
the alimentary canal of a bird is in itself a necessary preliminary to 
germination. 
My thanks are due to. Mr. E. H. Ellis of the Natural History Museum 
for his assistanee in connection with the work here described. 
* These fruits eventually germinated in July, 1924. 
