YELLOW GOAT's-BEARD. 51 



Acted on by light and moisture, the sap 

 deposits in its progress not only various se- 

 cretions of different qualities and tastes, but 

 serves also to perfect and to embellish the 

 plant through which it flows. Heat, too, is 

 a powerful agent in propelling its ascent, 

 acting as a stimulant, and essential in the 

 production of the smallest leaf or flower. 

 Mechanical causes also operate, such as the 

 frequent screw-like forms of the spiral ves- 

 sels : the motion that wind imparts even to 

 such plants as creep upon the ground, and 

 the passage of air through the conducting 

 vessels. All tliis is obvious, but why is it 

 that among the crowding flowers that skirt 

 the border of every corn-field, so great a 

 dissimilarity is perceptible ? They are all 

 rooted in the same soil : the same warm 

 sun shines upon them ; the same showers 

 descend to fertilize them : yet one is yel- 

 low, another red, a thu'd is lightly tinted 



