PLA 



PLA 



327 



The wood of trees is composed 

 of an external or living part, call- 

 ed alburnum, or sap'zoood, and of 

 an internal or dead part, the heart- 

 wood. The alburnum is white and 

 full of moisture, and in young trees, 

 and annual shoots, it reaches 

 even to the pith. The alburnum is 

 the great vascular system of the 

 vegetable through which the sap 

 rises, and the vessels in it extend 

 from the leaves to the minutest 

 filaments in the roots. 



The pith occupies the centre of 

 the plant ; its texture is membra- 

 nous ; it is composed of cells, which 

 are circular towards the extremity, 

 and hexagonal in the centre of 

 the substance. In the tirst infan- 

 cy of the vegetable, the pith 

 occupies but a small space. It 

 gradually, and in annnal shoots and 

 young trees, offers a considerable 

 diameter. In the more advanced 

 age of the tree, acted on by the 

 heart-wood, pressed by the layers 

 of the alburnum, it begins to di- 

 minish, and in very old forest- 

 trees disappears altogether. 



The leaves, though infinitely di- 

 versified in their forms, are in all 

 cases similar in interior organiza- 

 tion, and perform the same func- 

 tions. 



The alburnum spreads itself 

 from the foot-stalks into the very 

 extremity of the leaf; it retains a 

 vascular system and its living 

 powers ; and its peculiar tubes, 

 particularly the tracheae, may be 

 distinctly seen in the leaf. 



PLASTER OF PARIS. Great 

 differences exist among agricul- 

 turists respecting the uses of 

 gypsum, and the manner of its 



operation. Some have supposed 

 that its efficiency as a manure is 

 altogether owing to its power of 

 attracting moisture from the air. 

 But Sir Humphrey Davy is of 

 opinion, that but little effect can 

 be produced by such attraction. 

 " When combined with water," he 

 observes, "it retains that fluid too 

 powerfully to yield it to the roots 

 of the plant, and its adhesive attrac- 

 tion for moisture is inconsidera- 

 ble ; the small quantity in which it 

 is used likewise is a circumstance 

 unfavourable to this idea." Some 

 have supposed that gypsum assists 

 in promoting the putrefaction of 

 animal substances, and the decom- 

 position of the manure in the soil. 

 Sir Humphrey Davy, however, has 

 proved by repeated experiments, 

 that it rather retards than accele- 

 rates petrefaction. This philoso- 

 pher likewise says, " In examining 

 the ashes of sainfoin, clover and 

 rye-grass, I found that they afford- 

 ed considerable quantities of gyp- 

 sum ; and this substance, probably 

 is intimately combined as a neces- 

 sary part of their woody fibre. If 

 this be allowed, it is easy to explain 

 the reason why it operates in such 

 small quantities ; for the whole of 

 a clover crop, or sainfoin crop, on 

 an acre, according to my estima- 

 tion, would afford by incineration 

 only three or four bushels of gyp- 

 sum. The reason why gypsum is 

 not generally efficacious,is probably 

 because that most cultivated soils 

 contain it in sufficient quantities 

 for the use of the grasses. In the 

 common course of cultivation gyp- 

 sum is furnished in the manure ; 

 for it is contained in stable dung, 



