48 HEREDITY AND EVOLUTION IN PLANTS 



division, is a direct inheritance, but the chlorophyll subse- 

 quently manufactured, and the green color which it gives 

 to the plant, are not inherited; they are expressions of the 

 inheritance-T^which in this instance is a chloroplastid 

 (Fig. 34) that reproduces itself by division, and manufac- 

 tures chlorophyll in the presence of sunhght. Under abnor- 

 mal external conditions the mechanism may not act, or 

 may act abnormally, so that yellow pigment appears 

 instead of green — or in darkness no pigment at all. In 

 either case the inheritance is the same, but the expression 

 varies. A modern writer (J. Arthur Thomson) has defined 

 inheritance as all that an organism has to start with. It is 

 the protoplasmic substance, with all its potentialities, 

 passed on from parent to offspring. 



39. Inheritance Versus Expression. — In the light of 

 this information, obtained by a study of the simple Pleuro 

 coccus, we are able to understand that what we inherit 

 from our parents or grandparents, is not a certain shape of 

 nose, a certain characteristic gait, a musical or mathe- 

 matical bent of mind, a quick temper, but a substance 

 (protoplasm) possessing a very delicate, intricate, and 

 characteristic constitution or mechanism. Under certain 

 conditions this inheritance may so express itself as to 

 cause resemblance in some physical or mental trait; or it 

 may find a quite different expression, as when parents of 

 medium height have tall children, or parents musically 

 inclined have children that do not care for music; or sweet- 

 peas, having white flowers only, produce, when crossed, 

 peas having colored flowers. Or again, not all that is in- 

 herited may be expressed; this is illustrated when children 

 resemble, not their parents, but their grandparents. 

 Here the parents transmitted an inheritance which, in 

 them, found no expression. 



