io6 Introduction to Botany. 



outside world — light, heat, gravity, mechanical impact, 

 etc., being probably first perceived and communicated by 

 it to the other organs of the cell. 



The cytoplasm is the medium of interchange of stimuli 

 between the plasma membrane and the nucleus ; it prob- 

 ably manufactures various nitrogenous food materials from 

 substances furnished it by the chloroplasts, and from salts 

 of nitrogen, sulphur, and phosphorus, which have come 

 up from the soil. It appears to have something to do with 

 the production of ferments which render the reserve food 

 materials more soluble and diffusible ; it is probably con- 

 cerned with manifold changes of a chemical nature which 

 are constantly taking place within the cell; and it con- 

 tributes some of its own substance toward the production 

 of certain fibrillar structures which assist in nuclear and 

 cell division. 



The leucoplasts have the power of forming starch from 

 materials which come to them from the leaves ; or when 

 they are exposed to the light they may produce chlorophyll 

 within themselves, and become chloroplasts ; or they may 

 produce other coloring substances than green, as seen in 

 certain flowers and ripening fruits, and then they are termed 

 chromoplasts. Leucoplasts, chloroplasts, and chromoplasts 

 collectively are called plastids. 



To the nucleus, in particular, is intrusted the very impor- 

 tant function of bearing and bequeathing from generation 

 to generation the inheritable qualities. That the embryo 

 in an acorn shall develop into an oak instead of into an- 

 other kind of plant depends in large measure on the nuclei 

 in the cells of the embryo. The nuclei, as the bearers of 

 the inheritable qualities, must determine how the cells shall 

 behave under varying conditions. The nucleus has to do 

 also with processes involving chemical changes, such as the 



