ADDRESS. 21 



The colouring matter of chlorophyll presents under the spectroscope 

 a very characteristic spectrum. For our knowledge of its optical pro- 

 perties, on which time will not now permit me to dwell, we are mainly 

 indebted to the researches of your townsman, Dr. Sorby, who has made 

 these the subject of a series of elaborate investigations, which have con- 

 tributed largely to the advancement of an important department of 

 physical science. 



That the chlorophyll is a living substance, like the uncoloured proto- 

 plasm of the cell, is sufficiently obvious. When once formed, the chloro- 

 phyll granule may grow by intussusception of nutriment to many times 

 its original size, and may multiply itself by division. 



To the presence of chlorophyll is due one of the most striking aspects 

 of external nature — the green colour of the vegetation which clothes the 

 surface of the earth ; and with its formation is introduced a function of 

 fundamental importance in the economy of plants, for it is on the cells 

 which contain this substance that devolves the faculty of decomposing 

 carbonic acid. On this depends the assimilation of plants, a process 

 which becomes manifest externally by the exhalation of oxygen. Now it 

 is under the influence of light on the chlorophyll-containing cells that 

 this evolution of oxygen is brought about. The recent observations of 

 Draper and of Pfeffer have shown that in this action the solar spectrum is 

 not equally effective in all its parts ; that the yellow and least refrangible 

 rays are those which act with most intensity ; that the violet and other 

 highly refrangible rays of the visible spectrum take but a very subordi- 

 nate part in assimilation ; and that the invisible rays which lie beyond the 

 violet are totally inoperative. 



In almost every grain of chlorophyll one or more starch granules may 

 be seen. This starch is chemically isomeric with the cellulose cell-wall, 

 with woody fibre, and other hard parts of plants, and is one of the most 

 important products of assimilation. When plants whose chlorophyll 

 contains starch are left for a sufficient time in darkness, the starch is 

 absorbed and completely disappears ; but when they are restored to the 

 light the starch reappears in the chlorophyll of the cells. 



With this dependence of assimilation on the presence of chlorophyll 

 a new physiological division of labour is introduced into the life of 

 plants. In the higher plants, while the work of assimilation is allocated 

 to the chlorophyll- containing cells, that of cell division and growth 

 devolves on another set of cells, which, lying deeper in the plant, are 

 removed from the direct action of light, and in which chlorophyll is 

 therefore never produced. In certain lower plants, in consequence of their 

 simplicity of structure and the fact that all the cells are equally exposed 

 to the influence of light, this physiological division of labour shows 

 itself in a somewhat different fashion. Thus in some of the simple 

 green algas, such as Sjnrogyra and Hydrodictyon, assimilation takes place 

 as in other cases during the day, while their cell division and growth 

 takes place chiefly, if not exclusively, at night. Strasburger, in his re- 



