38 ON THE HIBERNA^TION OF LEMNACEJE. 



stagnant water they are even earlier ; in sliady places their production 

 is considerably delayed, or even completely suspended. These leaves 

 are for the most part somewhat reniforra, occasionally elliptical ; they 

 present no gibbosity, are flat, and of very small thickness ; of an olive- 

 brown colour on both sides. Their aeriferous system is very simple, 

 and causes them to resemble, up to a certain point, an ordinary bud 

 arrested in its development. The lacunae similar to those in the sum- 

 mer-leaves, do not exist ; the system consisting entirely of star-like pas- 

 sages. These winter-leaves are heavier than water, and descend to tbe 

 bottom as soon as any mechanical cause, such as rain or the agitation 

 of the water, causes them to become detached from the parent-leaf, 

 which they sometimes drag down with them. They pass the winter 

 in the mud, protected from the frost, and regain the surface only at the 

 period of the general awakening of vegetation. The first phenomenon 

 which marks their approaching ascension, consists in the appearance of 

 a bubble of oxygen on their upper surface. Lightened by the adhesion 

 of this bubble, the leaves reach the surface, whence they may descend 

 again should unfavourable weather occur, to rise once moi'e with a fresh 

 bubble, which iinally remains attached to them for a sufficient time for 

 the upper surface to become thoroughly dry, when the leaf assumes its 

 ordinary floating character. During the spring the leaves undergo 

 considerable changes, increase in size, the roots make their appearance, 

 and they become green. The complete aeriferous system then deve- 

 lopes itself, especially towards the upper surface, the true lacunse 

 making their appearance. Finally, about March, other leaves are pro- 

 duced from buds developed in their clefts. Although under ordinary 

 circumstances the summer-leaves perish with the production of the 

 winter-leaves, and frost is fatal to them, yet they occasionally live 

 through the winter till the month of February. Sometimes also this 

 species is propagated through the winter by floating leaves intermediate 

 in character between the two kinds described above. 



Lemna glhba. This species, like the preceding, possesses leaves 

 specially adapted for the winter. In favourable places they appear 

 towards the end of the summer, principally in September and October. 

 They are floating, flat, obovate, and remarkably symmetrical, of a violet- 

 green on their upper surface. They possess a free pendent root. Their 

 aeriferous system is remarkable from its imperfect development ; the 

 lacuufe are reduced to a rudimentary condition, or changed into simple 



