854 PHOTOLYSIS IN LEMNA TRISULCA. 



no evidence as to positive apostrophe (*.*., apostrophe caused by 

 strong light) being a natural event ; and when it is remembered 

 that L. trisulca usually, if not always, grows immersed to some 

 depth in the water of shaded ponds, we may well doubt whether the 

 indicated movement is of ordinary occurrence. Perhaps some 

 microscopist living near the habitat of this duckweed will en- 

 lighten us upon this point. 



It is, however, when nocturnal effects come under consideration 

 that current opinion most readily admits of challenge. St aid's 

 figure shows the chlorophyll of the thin part of the frond ranged 

 upon the side walls during the night, while in the cells of the thick 

 part the inner wall also is studded with chlorophyll, the superficial 

 wall being bare. According to Schimper,* however, while all the 

 grains upon the wall abutting upon the epidermis are apostrophised 

 during the night, a few of those ranged during the day upon the 

 inner wall still remain in epistrophe. 



If Stahl be correct, an ugly hole is made in a hypothesis for 

 which I am responsible. In my first memoir 4 it was shown that 

 sun-loving aerophytes differ photolytically from aquatics, inasmuch 

 as while the chlorophyll of the former is apostrophised during the 

 night, in the latter epistrophe obtains, negative apostrophe not 

 Betting in until after some days' or weeks' continuous darkness, and 

 an endeavour was made to explain this discrepancy on the view 

 that, aquatics being adapted to a lower intensity of light than 

 aerophytes, greater effect— in the form of mechanical strain— is 

 produced in their protoplasm, and so, longer time is required by 

 them in order that the motile effects of 'light may soak out. It 

 was explained that L. trimlca is a partial exception to this rule, 

 since most of the chlorophyll grains are apostrophised during the 

 night, although a few may still remain in epistrophe, even after 

 exposure to darkness for several days. On re-examining the 

 matter, however, I find that the exception is not so striking as it 

 appeared to be two years ago. I have thus the misfortune of being 

 again at variance with the respected German authority above- 

 mentioned. 



Wishing to get to the bottom of this question, I took the 

 trouble of making a number of calculations to determine, firstly : 

 the number of chlorophyll grains in the cells both of the thick 

 and the thm portions of the fronds ; secondly, the proportion of 

 apostrophised to still epistrophised grains in the cells of plants set 

 overnight m darkness ; and, thirdly, the further apostrophising 

 effect of a few days' seclusion from light. The number of grams 

 was determined upon specimens growing in light of intensity cor- 

 responding to that at a point well within the epistrophic interval ; 

 and since the result of a preliminary estimate taught that approxi- 

 mately the same number of grains were ranged upon both walls 

 parallel to the surface, only one wall was brought into focus, and 

 by doubling th number found thereon, the total was arrived at. 

 ilie following table, dealing with the marginal cells, requires no 



»nuu vxx^ iixun, 



Jahrb. f. Wi . Bot. 1885, p. 231. J Op. cit. 



