126 THE WONDERS OF VEGETATION. 



that they ha-ve misled many a naturalist and justified 

 the comparison of vegetable sap with animal blood. 

 Yet it seems that the term of " vital fluid," as applied 

 to the latter, is inappropriate, and that of milky sap 

 is more justifiable. 



Certain trees which contain milky sap in great 

 quantity have been called the serpents of the vegeta- 

 ble kingdom ; and the most striking feature of the re- 

 semblance is in the organ, by the help of which both 

 the plant and the animal emit poison. It is well known 

 that with many serpents the poison is held in two 

 long teeth of the upper jaw, which are traversed 

 throughout their whole length by a narrow canal. 

 At the root of these teeth is a gland that secretes the 

 poison, and can be compressed by the pressure of the 

 teeth like a sponge. At the moment when the ani- 

 mal bites, the poison is thrown into the medullary 

 canal of the tooth and through a small opening into 

 the wound. In poisonous plants we observe a simi- 

 lar arrangement in the bristles of the leaves we can 

 easily see this by examining the leaves of a nettle. 

 The poison of the common nettle is as little danger- 

 ous as that of many snakes, but it becomes deadly as 

 we approach the equator, the heat of the tropical sun 

 seeming to intensify the venom both of the plant and 

 the snake. 



The three great families which are distinguished 

 for the abundance and the value of their milky juices 

 resemble each other in the nature of that liquid ; and 

 hence we shall here mention only the most remarka- 

 ble species. Foremost among these stands a vegeta- 



