ON STEMS. 43 



The structure of exogenous plants or dicotyledons, to 

 which the tre<= of our temperate climes belong, is much 

 more complicated. Here, then, are two reasons for our 

 submitting them to a more accurate investigation. 



The stem is composed of two separate parts: the one 

 ligneous, the other cortical ; in other words, it is formed 

 of wood and bark. 



The wood consists in the first place of the pith, a soft 

 medullary substance, which occupies the centre of the 

 stem, and is almost always of a cylindrical form. This 

 soft pulpy body does not grow or increase in size with 

 the tree, but retains the same dimensions it originally had 

 in the young stem.* 



Caroline. I thought that it rather diminished ; for if 

 you cut a young branch or stem, the growth of one sea- 

 son, the pith is very considerable, while little or none is 

 to be discovered in the trunk of a full grown tree. 



Mrs. B. The pith which fills the shoot of one season 

 is scarcely perceptible in a large tree ; the quantity, how- 

 ever remains the same. Its dimensions may be contract- 

 ed by the pressure of the surrounding coats of wood, 

 which sometimes so condenses and hardens it as to pre- 

 vent its being distinguished from them. 



Some trees have a much greater quantity of pith than 

 others ; the elder-tree, for instance, abounds with it. 

 The quantity of pith in the branches depends also upon 

 their nature ; if the branch is barren, it contains much 

 more than if it is destined to bear fruit, but in the same 

 individual stem or branch the quantity never alters. 



* In the ash the pith is uninterrupted and compact, in the Garget 

 Phytolacca decandra, it is composed 'of transverse partitions inter- 

 secting the tube of the stem, which in that plant is unusually large; and 

 in some Thistles, it more resembles the web of a spider attached to the 

 Bides of the tube, but neither regularly disposed nor so large as to occupy 

 its entire cavity. In the Hemlock and other Umbelliferous plants it 

 forms a fine delicate lining, remarkable in some cases for its brilliancy, 

 and in many of the grasses it presents a similar appearance. But in the 

 Zizania or wild Rice, one of the largest grasses in New England, it 

 forms in addition to this lining, distant partitions which interrupt the 

 cavity of the stem. 



198. What is said of the structure of trees in temperate climes'? 

 199. Of what is their stem composed 1 ? 200. Of what in the first 

 place does the wood consist"? 201. What is said of the growth of the 

 pith'? 202. What does Caroline say of the size of the pithl 203. 

 How does Mrs. B. account for the pith appearing smaller in large 

 trees than in young branches'? 204. What trees have the greatest 1 

 205. On what does the quantity of pith dependl 



