ON THE HIBERNATION OF LKMNACEiE. 39 



passages. As with S. poli/rr/iiza, the cells of the parenchyma are 

 gorged with fecula, which imparts to them their higli specific gravity. 

 They remain, however, floating through the winter, and p'resent, until 

 the period of their death, very little change of form, except a slight 

 increase in the spring of their aerlferous system, disappearing about 

 May or June. They give birth, not to buds, but to leaves of a new 

 form, which may be termed spring-leaves, flat, but of a less regular 

 shape, closely resembling those of L. minor. The first heat of summer 

 changes them into the ordinary gibbous form of the summer-leaves. 

 These perish at the approach of winter, as soon as the winter-leaves 

 are produced, being very sensitive to frost. 



Lemna minor. This species does not possess any form specially 

 adapted for the winter, the differences by which the leaves of this sea- 

 son are distinguished from those of summer are inconsiderable, they 

 are somewhat smaller in size, and the lacunae of their aeriferous sys- 

 tem are less prominently developed. The winter-leaves are lighter than 

 water, and float on the surface; they are provided with roots of vari- 

 able length ; under the influence of frost, however, their aeriferous 

 system becomes filled with water, their tissue perishes, and they sink 

 to the bottom, carrying with them the young buds, which remain pro- 

 tected by the mud till the return of spring, when their activity returns, 

 and they rise again to the surface, quickly covering the water with a 

 green carpet. 



Staiirogeton trisulca. Like the last species, this is not preserved 

 through the winter by leaves of a special character. The leaves pro- 

 duced on the approach of winter participate in the general sleep of 

 nature ; their density is similar to that of the summer-leaves, but their 

 size and thickness are less. Prom various circumstances, probably in 

 great part mechanical, they are carried to the bottom of the water 

 during the winter, and become disengaged and released towards the 

 spring. They are, equally with the other species, susceptible to frost, 

 and require the protection of the mud for their preservation during the 

 winter. 



Wolffia arrJiiza. This species M. Van Horen has ouly had the 

 opportunity of observing under cultivation. The leaves found in winter 

 are scarcely distinguishable from those of summer either in form 

 or internal constitution. A portion of them remain floating on the 

 surface through the winter ; others undergo the submersion in the 



