3o6 LIFE : OUTLINES OF GENERAL BIOLOGY 



nitrogenous salts in the soil. Such were some of the steps towards — 

 we must still say towards — an understanding of photosynthesis. 



How then shall we broadly outline photosynthesis? It is the 

 utilisation of the radiant energy of red- orange-yellow light-waves, 

 shining through the plant's green pigment which, in some subtle 

 way, enables the living matter to build up organic carbon compounds 

 out of carbon dioxide and water. In ordinary land-plants the water 

 is absorbed by the roots, and in submerged plants over the whole 

 surface, but it also can be taken in by exposed aerial parts, as by 

 the delicate surfaces of liverworts and mosses, and in the perched 

 plants (epiphytes) that have no contact with the ground. The carbon 

 dioxide may enter along with the water, either in simple solution or 

 in loose chemical union (H2CO3). Soil-water also contains small 

 quantities of mineral salts, which have their necessary and subtle 

 r61e. 



The laboratories in which the photosynthesis takes place are the 

 "chlorophyll grains", small protoplasmic corpuscles or discs, 

 containing this green colouring matter, and technically called chloro- 

 plasts. They are semi-solid differentiations of the cytoplasm in 

 which they lie, but have been often described as multiplying by 

 division. In most higher plants they have a bun-like shape, which 

 may be modified as their position within the cell is adjusted to light 

 and gravity. In some of the Algae they have rather striking shapes, 

 sometimes like stars. They are tinged by a complex of chlorophyll 

 pigments — which are discussed in another section. In brief, there 

 seem to be usually two "chlorophyll-greens" which are essential, 

 and two "chlorophyll-yellows" which are ancillary. 



The energy utilised in the photosynthetic process is that of the 

 red-orange-yellow light-waves which are absorbed by the chlorophyll- 

 complex. The light is essential, for a green leaf cannot make carbo- 

 hydrates in the dark; and the chlorophyll is essential, for the 

 carbohydrates are only formed in the chloroplasts. What exactly 

 takes place is still unsettled among the biochemists, but it seems 

 probable that "the light is changed into electricity" — i.e. to wave- 

 lengths in another octave of the long gamut of electromagnetic 

 radiations or ether-waves. It is also probable that this electric 

 energy reduces the carbonic acid (H^COj) first to formic acid and 

 then to formaldehyde (HjCO), which is then compounded into more 

 complex carbohydrates, e.g. glucose (grape sugar) and saccharose 

 (cane sugar). In the reduction process oxygen is given off as a by- 

 product. It seems that only a small percentage, 0-5 to 3, of the 

 energy absorbed by the chlorophyll is used in photosynthesis: part 

 of it goes to warm the interior of the leaf, and thus to help in the 

 vaporisation of water — a very important process. The green plant's 

 power of changing the radiant energy of light into the potential 

 energy of carbon compounds commands our admiration, but it 



