367 
Sunlight upon Aquatic Plants. 
thawed out 1 . It is interesting to notice that, when in the 
motile condition, organisms appear to be more sensitive 
to cold (and to other injurious agencies as well) than when 
in the non-motile resting or vegetating condition. This 
may be partly due to the protecting cell-wall being thin 
or absent in the motile condition, partly to the more highly 
developed functional activities and special irritabilities which 
the motile zoospore possesses. In the motile and highly 
irritable condition the plant is able to avoid injurious 
agencies to a greater extent than it can when in the non- 
motile or fixed condition, and hence, in the former case, 
a development of high resistant powers is less necessary ; 
for the motile stages are, as a general rule, of only short 
duration, and are produced only when the external conditions 
are favourable. 
Even in multicellular plants, the protoplasts of which are 
always clothed with a cell-wall, an increased motility of the 
plasma seems to go hand in hand with a decreased resistant 
power to cold. In certain cases, at any rate, an increase 
in the amount of water present in the cell or cell-plasma 
accompanies this increased motility. This is the case, for 
example, in the rotating cells of Chara and Nitella , in the 
staminal hairs of Tradescantia , and in the parenchyma 
of the leaf and stem, and in the epidermal cells of E lode a 
canadensis and Vallisneria spiralis , in which various stimuli 
readily induce rotation. For in all these cases streaming 
movements appear only as the amount of water present in 
the cell increases, and the most active rotation is shown 
only when a single large central vacuole is present. No cell, 
the protoplasm of which shows active streaming movements 
or rotation, appears to be able to withstand either freezing 
or desiccation 2 . 
From the observations made by Messrs. West upon the 
survival of certain forms in frozen pools from year to year, 
1 Zoospores of Ulothrix and Haematococcus can continue to move at o° C. 
2 See Assim. Inhib., Journ. Linn. Soc. Bot., Vol. xxxi, p. 394; also Trans. 
Liverpool Biol. Soc., 1 897, p. 157. 
