224 PHARMACEUTICAL BOTANY 
The chief functions of a leaf are photosynthesis, assimilation, 
- respiration and transpiration. 
PHOTOSYNTHESIS is the process possessed by the cells of all 
green leaves or other green parts of plants of building up sugar, 
starch or other complex organic (carbon containing) substances 
by means of chlorophyll and sunlight. This process takes place 
in nature, only during sunlight but will also occur in artificial 
light. In this process CO, is taken into the leaf through the 
Fic. 153.—The end of a very small veinlet as seen in a section of the spongy 
parenchyma cut parallel to the leaf surface. The walls of the conducting cells 
of the veinlet have spiral thickenings. The border parenchyma cells surrounding 
the veinlets serve to conduct the sugar from the mesophyll to the phloem cells of 
the vein. (From Mottier, after Strasburger, Macmillan Co.) 
stomata and passes into the intercellular-air-spaces. between 
the chlorenchyma cells. It diffuses through the cell walls of 
these into the protoplasts where it passes into solution and enters 
the chloroplastids along with H:O from the soil. The chloro- 
plasts contain chlorophyll which absorbs the light energy. This 
light energy is then used either by the chlorophyll or, as seems 
most likely, by the whole chloroplast to unite the CO: with the 
H,O to form carbon compounds. These contain stored or poten- 
tial energy. ‘The kinetic energy of the sun’s rays thus becomes 
stored as potential energy and O; is given off. The plant gains 
