14 PHYSIOLOGY OF NUTRITION. 



4. - The Transparency of Plant Tissues. 



Light is of great importance in connection with very many 

 physiological processes in plants, and rays of unequal refrangi- 

 bility must by no means be considered equivalent as regards the 

 influence which they exert on plant life. It is therefore not with- 

 out interest to make some experiments respecting the transparency 

 of tissues. The depth to which rays of light penetrate into plant 

 tissues depends on the one hand upon the intensity and the 

 refrangibility of the light rays, and on the other upon the chemical 

 character of the constituent parts of the cells themselves, and the 

 anatomical structure of the tissues. As regards this last, the 

 presence of a more or less extensive intercellular system inter alia 

 plays an important part. If, e.g., many intercellular spaces occur, 

 the incident rays will be obliged to pass very frequently from the 

 cell sap, and through the cell walls with their imbibed water, into 

 air, and this naturally will greatly diminish the transparency of a 

 tissue. The significance of the intercellular spaces in this respect 

 will at once be manifest, if we make the following experiment:. A 

 piece of Begonia manicata leaf is laid in water, contained in a 

 small glass. We close the mouth of the glass with a rubber 

 stopper, through which has been passed one end of a glass tube 

 bent at right angles, the other end of this tube being connected 

 with an air pump. On exhausbing, air escapes from the inter- 

 cellular spaces ; these however fill with water, and the leaf now 

 appears far more transparent than at the commencement of the 

 experiment. If we immerse the blade of a leaf of Primula sinensis 

 in water, take the end of the leaf-stalk in the mouth, and remove 

 air from the intercellular system by sucking, water penetrates into 

 the intercellular spaces of the leaf through the stomata, and the 

 leaf is thereby rendered fairly transparent. 



Cork tissue, owing to the specific nature of its cell contents, is 

 only slightly transparent. Similarly tissues rich in chlorophyll 

 absorb much light owing to the presence of the pigment, and 

 transmit only comparatively little. Here also it may be mentioned, 

 although we shall consider the matter in more detail later, that 

 the chlorophyll pigment has the power of absorbing very ener- 

 getically the so-called chemical rays. We can easily prove this by 

 laying any green leaf on a piece of photographic paper, and 

 exposing between two sheets of glass to the influence of the light. 

 The portion of the paper not covered by the leaf rapidly becomes 



