44 ON STEMS. 



The pith consists of cellular tissue. If this membrane 

 be of a very fine texture, it is susceptible of extension as 

 the branch lengthens ; but if it be coarse, and the cells 

 large, when the branch grows, it cracks and separates in- 

 to parts. This is distinctly visible in a branch of jessa- 

 mine, if you slit it open so as to exhibit the pith.* 



Here is one which we may examine. I will slit it lon- 

 gitudinally : look, Emily, the pith is separated into parts, 

 as if it had been forcibly torn asunder. 



Mrs. B. It is the growth of the stem which thus 

 rends the pith in pieces. 



Emily. Then is it not destroyed and rendered useless ? 



Mrs. B. Yes ; but not until it has fulfilled the purpose 

 of its destination, which is to nourish the young wood 

 during the first period of its existence. 



Emily. It acts the part of a cotyledon or nurse to the 

 young wood. But when it is become dry, what is to per- 

 form this office to the new wood which is annually formed? 



Mrs. B. Every new layer of wood is lined with a lay- 

 er of cellular tissue, which may be considered as the pith 

 of the wood to which it is attached. These internal 

 coatings not only separate the several layers of wood, but 

 are also interwoven and incorporated with them, and may 

 be seen in the form of rays, which appear to issue from 

 the central pith, and proceed to the external layer of 

 wood : these are called medullary rays ; they are visible 

 in wood, but are remarkably distinct in the root of the 

 carrot. 



Caroline. But these fibres or rays, which appear all 

 to proceed from the centre, cannot be continuous, since 

 they originate annually in each fresh growth of wood. 



Mrs. B. Very true ; but they are so minute and so 

 numerous, that the termination of those of one year's 

 growth, and the commencement of those of the follow- 

 ing year 3 cannot be distinguished. This gives them the 



* When young the pith is usually green, becoming of a snowy white- 

 ness when old, though in some few cases it becomes yellow or brown. 



206. Of what does the pith consist! 207. What is said of the 

 pith of jessamine 1 208. What is the use of pith! 209. With 

 what is every new layer of wood lined! 210. What is said of the 

 separation of the layers of wood by these internal coatings'? 211. In 

 what are these coatings remarkably distinct! 212. Why does Caro- 

 line say these rays cannot be continuous! 213. Why does Mrs. B. 

 say the termination of growth one year cannot be distinguished from the 

 beginning of growth the following year! 



