176 Chapters in Modern Botany CHAP. 



the veins are more than supporting beams ; they include a 

 system of downcast pipes by which the sugars and other 

 materials manufactured by the leaves pass away into the 

 stem, and another upcast system of pipes, whose necessity 

 is equally obvious, by which the water and soluble salts 

 absorbed by the roots and passed up the stem irrigate the 

 leaf and supply some of the essential raw materials for the 

 manufacture which goes on there. These two systems 

 bast and wood respectively may be more fully studied when 

 we trace them into the stem. 



Palisade Cells and Chlorophyll Grains. To the ordi- 

 nary observer it seems as if the chlorophyll grains of the 

 palisade cells had no definite position ; but Stahl and others 

 have made the very interesting discovery that a movement, 

 at any rate a change of position of the chlorophyll grains 

 within the living cells, may be observed when cells are 

 brought from diffused light into intense sunshine. The 

 leaves of the Star of Bethlehem (Ornithogalum umbellatum 

 and O. nutans), of grape hyacinth {Muscari racemosuni), and 

 other common plants, notably also the lesser celandine or 

 figwort {Ranunculus Ficaria), give excellent examples of 

 this, as also do the prothallia of ferns (although here, of 

 course, a palisade parenchyma is not specialised). In 

 figwort, for instance, the chlorophyll grains will leave the 

 upper and under surfaces of the palisade cell for its sides after 

 quarter of an hour of sunshine, thus taking up a position in 

 which they are as little as possible exposed, not only sheltered 

 one behind the other, but with their edges to the light. When 

 the sun goes off, they get back to their old positions, but 

 more slowly, needing several hours. This is, of course, 

 observed by cutting sections of leaves taken at different 

 times. 



Now we know that too much light may be injurious ; 

 and Pringsheim has made striking experiments towards 



