CHRMICiAL AND PHYSIOLOGICAL STUDIES. 167 



chlorophyll of certain light-rays into energy of chemical action 

 during the processes of starch-formation ; and in this process, as 

 has been seen, CO, and water are assimilated, and certain inter- 

 mediate carbon compounds are formed, to be quickly broken 

 down or synthesised again (photosynthesis). It is probable that 

 the red rays (the least refrangible) arc the ones most utilised in 

 these reactions, and since the more refrangible violet and blue 

 rays are also absorbed to a certain extent, it seems likely that 

 they are also used to further the process. Possibly these rays 

 are converted into others of a lower refrangibilit}-, or, as is more 

 rational to suppose, the red raj's may be converted into those 

 which are known to further certain chemical actions {ef. infra, 

 Timiriazeff's experiment). 



If green parts of plants are kept in the dark for some time 

 they become etiolated — that is, the chlorophyll disappears (or 

 etiolin is formed), and the chloroplasts are unable to assimilate 

 carbon dioxide and water and produce sugar to the same extent 

 as before. In the case of plastids, in parts of plants which are 

 not green, formation of starch probably takes place by the con- 

 version by these structures of elaborated material (sugar) from 

 the leaves into other carbon-compounds, and finally into starch 

 (chemosynthesis). In some cases, also, it is probable that the 

 cytoplasm undergoes a gradual conversion into starch by the 

 splitting off of the proteid and amine portions of its molecule. 



Chlorophyll has been found in those parts of plants which 

 have never been exposed to the light, such as the seed-leaves of 

 Pinus, and in the phelloderm formed from the cork-cambium 

 in certain stems (see Chap. iii.). In such cases it is probable 

 that chlorophyll is formed in a somewhat different manner to 

 that in which it arises in parts exposed to the light, or else that 

 just enough light penetrates to these tissues for the pigment to 

 be formed.* 



(6) Pigments other than Chlorophyll occurring in Plant-cells. 

 — Red, blue, and yellow colouring matters may exist in cells, 

 singly or combined, either in the form of chromoplasts contain- 

 ing the pigment, or dissolved in the cell-sap. In the cells of 

 the petals of Tropmolum, angular chromoplasts are to be 



* It can be demonstrated that the light needed for chlorophyll-forma- 

 tion need not necessarily be so intense as that needed for CO2 assimilation 

 (see Darwin and Acton, Practical Plant Physiology). 



