164 



BAP. 



the new fixed properties, the race becomes deteriorated, in 

 proportion to the want of care that has occurred, and loses its 

 characters of individuality. 



255. In all varieties those seeds may be expected to pre- 

 serve their individual characters most distinctly which have 

 been the best nourished (243.) ; it is, consequently, those 

 which should be selected in preference for raising new plants, 

 from which seed is to be saved. 



256. When seeds are first ripened, their embryo is a mass 

 of cellular substance, containing- starch, fixed carbon, or other 

 solid matter in its cavities ; and in this state it will remain 

 until fitting circumstances occur to call it into active life. 



257. These fitting circumstances are, a temperature above 

 32° Fahr., a moist medium, darkness, and exposure to air. 



258. It then absorbs the moisture of the medium in which 

 it lies, inhales oxygen (278.), and undergoes certain chemical 

 changes; its vital powers cause it to ascend by one extremity 

 for the purpose of finding light, and of decomposing its carbo- 

 nic acid (279.), by parting with its accumulated oxygen, and 

 to descend by the other extremity for the purpose of finding a 

 constant supply of crude nutriment. 



259. Unless these conditions are maintained, seeds cannot 

 germinate ; and, consequently an exposure to light is fatal to 

 their embryo, because (278.) oxygen will not be absorbed in 

 sufficient quantity to stimulate the vital powers of the embryo 

 into action, for the purpose of parting with it again, by the 

 decomposition of the carbonic acid that has been formed dur- 



ing its accumulation. 



X. Sap. 



260. The fluid matter which is absorbed either from the 

 earth or from the air is called sap. 



261. When it first enters a plant it consists of water holding 

 certain principles, especially carbonic acid, in solution. 



262. These principles chiefly consist of animal or vegetable 

 matter in a state of decomposition, and are energetic in pro- 

 portion to their solubility, or tendency to form carbonic acid 

 by combining with the oxygen of the air. 



263. Sap soon afterwards acquires the nature of mucilage 

 or sugar, and subsequently becomes still further altered by 

 the admixture of such soluble matter as it receives in passing 

 in its route through the alburnum or newly-formed woody 

 tissue. (65.) 



264. When it reaches the vicinity of the leaves it is attracted 

 into them, and there, having been exposed to light and air, is 

 converted into the secretions peculiar to the species. 



265. Jt finally, in its altered state, sinks down the bark, 

 whence it is given off laterally by the medullary rays, and is 

 distributed through the system. 



