26 LIFE AND LIGHT 



positive results of such tests after photo- synthesis may be due to 

 formaldehyde, this does not prove that formaldehyde is the main 

 primary product of photo-synthesis, and it may only be produced 

 as a by-product in minute traces in secondary metabolic reactions. 

 It is in fact only in the minutest traces, the upper limit being about 

 0-001 per cent., that formaldehyde, which is one of the most intense 

 of protoplasmic poisons, can be borne by living cells. Direct ex- 

 periments with less than the lethal dose of formaldehyde in regard 

 to photo- synthesis have given negative results; plants subjected in 

 presence of light to solutions containing 0-001 per cent, of formal- 

 dehyde do not form as much starch as in its absence. 



Again, starch granules appearing in the cell are not always the 

 first visible sign of photo- synthesis, although the most frequent, 

 for in certain plants the starch granules are replaced by oil droplets. 

 Also, when we begin with the etiolated cell, we see that chlorophyll, 

 the green colouring matter itself, which is a very complex nitrogen- 

 ous organic body, is formed under the influence of light in the 

 plastid, and that under the influence also of light the protein matter 

 of the plastid increases in amount. 



It hence becomes probable that carbohydrates, proteins, and 

 fats, as also chlorophyll, a body of lecithide nature, are all formed 

 in the plastid, and accordingly it appears likely that the carbon 

 dioxide, water, nitrates, etc., required for the building up of all these 

 bodies should first form an integral part of the plastid, and then, 

 after synthetic processes in which the complicated biotic molecules 

 of the plastid take a part, be detached as proteins, lecithides, fats, 

 or carbohydrates. 1 



Theoretically, the supposed formaldehyde production rests upon 

 an exceedingly crude basis of organic chemistry and the experimental 

 evidence on behalf of it, in spite of much most earnest and pains- 

 taking work, is of the flimsiest character. 2 Sugars are aldehydic in 

 character, and if we reduce the formula of a hexose to its simplest 

 form by dividing by six it becomes CH.,0, which is the rational 

 formula of formaldehyde ; also by condensation of formaldehyde it 

 has been shown that a mixture of hexoses containing fructose can be 



1 A considerable amount of evidence has recently been accumulated by 

 Parkin to show that cane-sugar is one of the earliest carbohydrates to make 

 its appearance in the green leaf. 



2 See, however, succeeding experimental work detailed in Chapters III., 

 IV., and V. of this book. 



