Lecture XIX. 157 



current which it produces in the tracheids is called the transpira- 

 tion-current. It is evidently of great importance in bringing a 

 supply of water and dissolved mineral substances to the mesophyll 

 cells. The water thus supplied enables these cells, containing 

 chloroplasts, surrounded with air and exposed to light, to manu- 

 facture carbohydrates. In the dissolved substances carried by 

 the transpiration-current these cells also find the remaining ele- 

 ments needed for the building of protoplasm, and it is highly 

 probable that the mesophyll cells are the laboratories in which not 

 only carbohydrates but also proteids and protoplasm are synthe- 

 sised. 



After the leaf has attained its definite size, the indefinite re- 

 tention and accumulation of these substances in it would serve no 

 purpose. They are required, however, at the growing regions of 

 the stem and root where new cells are being formed and tissues 

 differentiated. The transport of these materials to the growing 

 regions is usually assigned to the bast, or phloem, of the con- 

 ducting tracts and more particularly to the sieve-tubes. The 

 mechanism of their transport is, however, unknown. Their presence 

 in the sieve-tubes may be detected ; and by breaking the conti- 

 nuity of the bast, growth at the tips is suspended, but further than 

 this little is known of the process. 



The solution absorbed from the soil is very dilute and to obtain 

 sufficient quantities of the necessary materials transpiration and 

 evaporation are evidently of importance. These processes have 

 also another essential r61e. For the purpose of photosynthesis 

 leaves must be displayed to the light and are often unavoidably 

 exposed to direct sunlight. The light absorbed being largely 

 converted into heat would lead to a rise of temperature beyond 

 the limits of resistance of the cells, were it not that evaporation 

 cools the leaf. This function may be readily appreciated by 

 exposing side by side in the sun a living leaf with a piece of dry 

 but equally light-absorbing material or a dead dry leaf. The 

 living leaf will remain cool to the touch while the others become hot. 



When the leaves of the fern are exposed to light, the process of 

 photosynthesis liberates oxygen among the green cells of the 

 plant, and part of this oxygen is used in respiration by those cells. 

 In darkness when oxygen is not produced within the green tissues 

 and always in the colourless tissues of the stem and root, oxygen 

 must be conveyed to the cells. For it is easy to show that 

 Aspidium cannot continue to respire anaerobically without injury. 

 This supply of oxygen finds its way to the cells throughout the 

 plant by means of the air-passages and intercellular spaces in the 



