90 



Introduction to Botany. 





are represented in the diagram. The water from the roots 

 ascends the water tubes and passes by osmosis into the 

 paHsade cells and mingles with their sap. Since the sap 

 bathes the cytoplasm which lines the cell wall, the water 

 has easy access to the chloroplasts embedded in the cyto- 

 plasm (see page 36). The air with its carbon dioxide 



enters through the sto- 

 mata and fills the spaces 

 between the paHsade 

 cells, and needs only to 

 penetrate through their 

 thin walls in order to 

 come in contact with the 

 chloroplasts. 



The energy from the 

 sun is readily transmitted 

 through the epidermis to 

 the chloroplasts, and the 

 work of food making is 

 begun. Under these conditions, the chloroplasts may be- 

 come filled with starch in the course of an hour (see 

 Fig. 38). The starch, which is the first food substance 

 formed that can be detected by aid of the microscope, does 

 not long remain in the chloroplasts, but is made soluble, 

 mainly in the form of glucose or grape sugar ; and in this 

 form, or in combination with compounds of nitrogen and 

 sulphur to form proteids, it passes into the sieve tubes, 

 and is carried down the stem and in part into the roots, 

 or up the stem where buds are unfolding or flowers and 

 fruits forming, being drawn from the sieve tubes and 

 used for food wherever it may be needed throughout its 

 course. 



In the night, when the leaf can no longer obtain energy 



Fig. 38. 



Cross section through leaf of Melilotus alba 

 taken at sundown. The section has been 

 treated with chloral hydrate iodine, and the 

 dark granules in the cells are starch. 



