80 FAMILIAR LESSONS IK BOTANY. 



It seems evident that their generation must be like their 

 emission, periodical; but further than this no satisfactory 

 theory has been given. The law that controls them is hid 

 from us. Of course, various theories have been suggested. 

 The effects of solar light and heat, or its absence, and cer- 

 tain atmospheric conditions, have been referred to as causes. 

 Some have supposed that intervals of fragrance were caused 

 by the respiration of plants. 



208. We heard of the night-jasmine and its periodic 

 odor in our conversation about night-blooming flowers. 

 The day-jasmine, an equally familiar plant, noted also for 

 its intermittent odor, is quite scentless at night, but deli- 

 ciously fragrant in the day. The magnolia glauca gives 

 out great weaves of perfume at irregular intervals. 



209. N"o satisfactory theory has yet been presented to 

 account for these phenomena. To account for the other 

 properties, taste, texture, etc., is a much easier task. You 

 know that the flavor of vegetables varies with the modes 

 of culture. Not that cultivation will make a tomato pun- 

 gent like a pepper, the aloe sweet, or camomile sour. The 

 nature of the plant is fixed as to taste or odor, but the 

 manner of culture very materially governs quantify. Every 

 child who has cultivated his little "patch" of sugar-cane 

 knows that if he wants siueet cane he must have it grow in 

 the sunshine. 



210. The influence of light and heat, then, is to intensify 

 the peculiar properties of plants. Consequently the ab- 

 sence of these two solar influences will diminish them. 

 Celery, Avhen perfectly developed, is a pungent, disagreeable 

 plant. When its pungency is decreased by the absence of 

 light, the aromatic property is subdued into a delicate 

 flavor that makes the plant an agreeable salad. 



208. What 13 said of the night and day jasmine? Magnolia glanca ? 



209. Has any satisfactory theory been given of these phenomena ? 



210. What is the effect of light and heat on plants ? Example ? 



