58 SECRETIONS. 



sometimes resinous, sometimes composed of essential oils, and at 

 other times formed of fatty matters. 



58. The milky juices are chiefly found in the bark, and appear 

 to constitute the liquid we see circulating in the vessels of the 

 latex, in a great number of plants. The white liquid that runs 

 from the fig tree when it is cut, opium, caoutchouc (India rubber), 

 ^c., are juices belonging to this class. 



59. The resinous juices are very common in the bark, and are 

 also met with in other parts of the stem; they are formed in 

 little masses which become united together, and descend by their 

 own weight in the tissue of the plant. Sometimes these juices 

 are so abundant that, by making an incision in a tree, we cause 

 a stream to flow out of it, and in this way collect considerable 

 quantities of its proper juices ; as we see in pine and fir trees. 



60. The essential or volatile oils are contained in cells or 

 vesicles, and are found in the foliacious and cortical parts of 

 plants. And the proper juices constituted of fatty oils are chiefly 

 found in the seeds. 



61. The solid matter, found in the elongated cells of the wood, 

 and on this account called lignin (from the Latin, lignum, 

 wood), may also be considered as being the product of a species 

 of secretion, as well as the fecula, which is produced in great 

 abundance in certain parts of plants, seemingly forming deposits 

 of nutritive matter, destined at a future time for the nourishment 

 of the plant. This last substance has the appearance of small, 

 white, hard grains, which seem to be composed of different layers, 

 the exterior of which are hardest, and the most internal are simi- 

 lar to gum. It is found isolated in the cells of the cellular tissue; 

 and in some parts of certain plants, such as the seeds of wheat 

 or of rye, the tubers of the potatoe, the ligneous stems of mono- 

 cotyle'donous plants, &c., it forms considerable masses. 



OF THE GROWTH OF PLANTS. 



62. The growth of plants depends upon two phenomena : 1st, 

 the increase of the diameter of stems already formed ; 2d, the 

 development and elongation of new branches. We will succes- 

 sively examine both. 



58 Where are the milky juices found ? Give some instances of milky 

 juices. 



59. How are resinous juices collected from plants ? In what p irt of the 

 plant are they found ? 



GO. In what parts of plants do we find the essential oils ? In what part 

 Ihe fatty oils ? 



61. VVhat is lignin ? What is fecula ? Where is it found ? 



62. Upon what does the growth of plants depend ? 



