40 NUTRITION. 



soon dry up and split. This* difference in the 

 mode of growth accounts for the different re- 

 sults of such experiments in this part of the 

 tree, as were before mentioned as having been 

 tried in the wood if an inscription be made on 

 the bark only, the letters without lengthening, 

 gradually become thicker, larger, further apart, 

 and are at last effaced. The secretions of a 

 plant are often deposited in the bark. 



30. The Medullary Rays, formed of com- 

 pressed parallelograms of cellular tissue, connect 

 the centre and circumference of the trunk : they 

 strengthen the tissue, and convey secreted nu- 

 tritive matter in a horizontal direction. They 

 are distinctly perceptible in a section of a woody 

 stem. Sometimes they can be traced from the 

 central pith to the extreme circumference, but 

 ordinarily the line is interrupted. 



31. Stems vary extremely in appearance in 

 different plants sometimes they run under the 

 ground, and are improperly called creeping 

 roots; occasionally they lie prostrate, and send 

 roots into the earth underneath them ; the 



* It will be therefore observed tbat, strictly speaking, 

 it is the woody portion only of Exogenes to winch the 

 term applies, as the bark follows the laws of the Endo- 

 genous tribes. 



