DISPERSAL BY WATER. 



805 



called Pediastrum, which is closely related to the Water-net, and of which one 

 species is shown in figs. 370 ^■''•s. p. 640. 



The distribution of bud or sprout-like offshoots is seen especially in the Duck- 

 weeds, Alismacese, Potamogetons, Utriculariaceffi, Droseraceee, and Primulacese. 

 Most of the Duckweeds (e.g. Lemna polyrrhiza and L. arrhiza), which float during 

 the summer on the surface of still water, towards the autumn form organs on their 

 flattened stems which become detached from the summer plants, sink to the bottom 



Fig. 448. — Frogbit (Hydroeharis Morsus-ranoB). i Winter buds rising to the surface in spring. 2 Young floating plants 

 which have developed from such buds, s Older floating plants. 



of the lake, and stay there during the winter. Each of these organs is pocket- 

 shaped, and in the hollow the next year's shoot is already laid down — of course, as 

 a minute structure whose semicircular free end scarcely projects above the closely- 

 adjoining edges of the pocket. These detached winter buds sink because their cells, 

 even those of the epidermis, develop large starch gi-ains which are crowded together, 

 and literally fill up the lumens of the cells. There are no air-spaces like those which 

 cause the summer-plants to float on the surface of the water; the stomata as yet are 

 closed, and the whole body, hermetically sealed from the outer world, now has a 

 specific gravity which causes it to sink down to the bottom of the water, where it 

 is protected against the frost. There it remains in a resting condition during the 

 winter. At the beginning of the warmer season the bud wakes from its winter 



