President's Address. 347 



formed may be converted into colouring matter, and may thus be 

 the mother substance of the chlorophyll colouring matter, an hypo- 

 thesis which, as will presently be pointed out, may explain the 

 development of green colour in the dark within the tissues of 

 gymnospermous embryos. 



The relation of hypochlorin formation to assimilation and respira- 

 tion may be further elucidated by placing the seedlings grown in 

 the dark not in bright full daylight, but in conditions of half dark- 

 ness (a darkened room). Here they become quite green, but do not 

 live long, and disintegrate almost as rapidly as those growing in 

 complete darkness. Such facts have led to the supposition which 

 has been already refuted in this paper, that a plant becomes green in 

 light of a lower intensity than is requisite for assimilation. The ex- 

 planation really is that, in the relations of assimilation to respiration, 

 this low intensity is unfavourable for assimilation, and the products 

 of this process are, without any permanent gain to the plant, again 

 used up. If seedlings grown in darkened rooms are brought into 

 bright light, which is favourable to development of green colour, 

 but not to evolution of free oxygen, then no hypochlorin is to be 

 found in them. Such plants grown in half-dark conditions (whether 

 they have been in half darkness throughout, or were at first in 

 complete darkness) show no trace of hypochlorin, even if the plant 

 is as well formed and as deeply green as a seedling which has for 

 some days grown in full daylight and contains abundance of hypo- 

 chlorin. It depends on the regulation of the brightness whether or 

 no after a time, say eight to fourteen days, hypochlorin is found in 

 plants grown under half-dark conditions ; for as soon as assimilation 

 is greater than respiration, then hypochlorin accumulates and in- 

 creases in amount with the increasing brightness. Beautiful green 

 seedlings destitute of hypochlorin may be grown under a glass shade 

 covered with grey paper in the half -dark illumination on the side 

 farthest from the light of a deep room. Its absence from these is a 

 proof that it is used up in respiration so long as that process in light 

 is in excess of assimilation. Hypochlorin formation, therefore, is 

 fully proved to depend upon light. Its dependence upon the 

 presence of carbonic acid is difficult of experimental proof, because, 

 though seedlings may be cultivated in an atmosphere deprived of 

 carbonic acid, it is impossible by absorptive agents to keep it free 

 of the gas ; as in presence of oxygen, which is here necessary for 

 the development of the green colour in the tissues, the time that 

 must elapse before the hypochlorin formation can be detected is 

 sufficient for carbonic acid to accumulate within the tissues, and to 

 so great an extent as to give rise to hypochlorin. 



Of very striking import is the fact that hypochlorin, like chloro- 



