CHAP. H.] VEGETAL NUTRITION. 93 



the darkness, remain yellow, but under the action of even a 

 feeble light and a somewhat elevated temperature, of from 20 

 to 30 degrees, they grow green. All the rays of the solar 

 spectrum suffice to make the colourless chlorophyllian particles 

 green, but by far the. most active are the yellow rays. 



Once formed, the chlorophyllian particles, if they are favoured 

 by good conditions, increase in size, and at a given moment can 

 multiply by binary division. The solar light does not merely 

 influence the formation of the chlorophyllian particles ; their 

 whole evolution is subjected thereto. The particles veridified and 

 subjected to an intense light during a long space of time form 

 in their very substance particles of starch, which are most 

 manifestly the result of a nutrition too active, an aliment in 

 reserve. This starch besides re-dissolves and re-forms according 

 as the green cell is withdrawn from the solar light or exposed to 

 it anew. From a long sojourn in darkness the particles of 

 chlorophyll themselves lose their shape, suffer atrophy, and 

 disappear, dissolving into the colourless protoplasm. 



We have seen that all the rays of the solar spectrum have the 

 power to render green the chlorophyll ; but all are not capable 

 of impressing on it enough of nutritive activity to form the 

 starch in its particles. That is a faculty limited naturally to 

 the rays the most stimulating, the yellow rays. 



Light being the determinating agent of the chlorophyllian 

 formation, it is natural for the chlorophyllian particles to 

 accumulate specially on the best illuminated cellular wall ; and 

 i this, in reality, is what takes place. 1 



In the persistent leaves, heat appears to have a great influence 

 on the position of the particles of chlorophyll. In effect, when 

 the temperature lowers they quit the wall to accumulate inter- 

 . volved in the centre of the cell. In spring, or whenever the 

 plant is subjected to a certain elevation of temperature, they, 

 whether in darkness of in light, resume their parietal position. 

 Lastly, toward the end of the vegetative period, the precious 

 1 Franck, Botanische Zeitung, 1872. 



