84 Ver/etahle Phijsiohgy. 



that tlie growth occurring in the ahsence of light produces wealCy, 

 Avatnry textures, while the supply of light, ceteris paribus, is 

 marked by a corresponding solidity and vigour of the develop- 

 ment. We might figuratively say that plants supplied with 

 water and nitrogenous food, and withheld from light, are like 

 animals fed on substances deficient in nitrogen, which, as is. 

 often seen in badly-fed children, results in unhealthy fat and 

 want of vigour — since in animals the ternary compounds are 

 applied more especially to purposes of respiration or formation of 

 fat, while the nitrogenous compounds are required to form muscle 

 and solid structure. In plants the water and nitrogenous food seem 

 to favour expansion and development of new structure, while car- 

 bon, Avhich apparently can only be assimilated by the help of 

 the sun's rays, is the great element of the solid substance. 



Nevertheless, the nitrogenous part of the plant, the formativ^ 

 protoplasm contained in the cells, still maintains its place as the 

 living substance, when wc endeavour to follow out the changes, 

 iinatomical and chemical, taking place under the action of light. 

 If we examine into the cause of the green colour assumed by the 

 leaves and stalks of plants when exposed to light, we find it to 

 reside in granular structures or substances belonging to the proto- 

 plasmic cell-contents, which assume a green hue in consequence 

 of a chemical change effected through the agency of light. The 

 nature of this change is still unknown, but that the chlorophyll 

 consists of the protoplasmic matter coloured green, is certain. 

 Under a prolonged action of the chemical influence of light the 

 protoplasm goes on to secrete starch-grains, the more solid form 

 of assimilated matter of ternary composition, and the produc- 

 tion of the liqnine (woody) condition of cellulose, the substance 

 which forms the hard cell-walls, giving to the originally succulent 

 tissues the firm character of wood, is a result ot the modification 

 of the secreting action of the protoplasm dependent on the in- 

 fluence of light. The chemical actions which must occur in 

 these changes are at present very imperfectly understood. It is 

 only within a few years that the internal condition of the struc- 

 tures of plants has been thoroughly studied, and hitherto the 

 chemical inquiries have been made, as we may say, " in the 

 rough." The observations of Mulder on the yeast-plant are 

 almost the only examples of a thorough examination of the details, 

 in such inquiries ; in them the cell-membranes and the contents 

 were subjected repeatedly to analysis, the c;omparative accuracy 

 of the separation l^eing ascertained by the help of the microscope, 

 and the plants were exam hied microscopically in different stages 

 of growth, while the chemical change in the medium and in their 

 composition were ascertained by chemical analysis. But to 

 apply similar researches to the higher plants, those growing in 



