1849. 



THE GENESEE FARMttll. 



171 



BOTANY. 

 STRUCTURE AND FUNi I ill' STEM. 



Plants an o exogenous and endogenous. 



T:> row by accumu- 



lation, or layers of matti r from the outside. This 

 class includes m and most shrubs 



and herbaceous plants of temperate climates. 



The endogenous plants hich grow from 



the inside, or by accretion of matter within that al- 

 read] I. Most of the bulhous plants of tem- 



perate regions, all the grasses, and the ; alms, cane, 

 kc, of tropical countries, are endogenous. 



'I'h.' exogenous Btem consists of bark, vrood and 

 pith. 



The pith is a light spongy substance, at the centre 

 of the stem; it is composed of cellular tissue, and 

 exercise its peculiar functions only during 

 the earlier growth of plants. (Wood.) 



The wood is composed of cylindrical or concentric 



, intersected by medullary rays, which are those 



thin dense | lates of wood dividing the "grains," and 



rge and easily seen in a piece of beech or oak' 



Fig. 1. 



Fit;. 2. 



wood which has been 

 split. The pith, together 

 with the first layer which 

 incldses it, ar.e the pro- 

 duct of the first year's 

 growth ; one new layer 

 is formed every succeed- 

 ing year, — so that the 

 number of r i n g s o-r 

 " grains" at the base of 

 the stem indicate correct- 

 ly the age of the tree. — 

 Each layer is composed of 

 woody fibres, vasiform tis- 

 sue and ducts. 



| Fig. 1 represents an exoge 

 nn is stem of 1 year's growth; 

 a, pith ; I), bark ; c. medulla- 

 ry rays : d, woody bundles of 

 6bre. Fk'. 2, latieiferous ves- 

 sels of the bark.] 



The outside, lighter colored layers constitute the 

 albin :nm or " sap wood ;" the brownish layers inside 

 are harder than the sap wood, and are hence called 

 the duramen m 



The bark r irms the external covering or integu- 

 ments of the s'em and root. The bark consists of 

 throe distinct layers: the outside covering is called 

 the epidermis, — this layer is sometimes covered with 

 a coating of gummy, oily or resinous matter. The 

 middle layer is the cellular integument; and the inner 

 coat the liber. The two outer layers are of cellular 

 structure, while the inner one is both cellular and 

 woody. 



The sap is carried by the vessels through the al- 

 burnum to the leaves, with the vessels of which they 

 communicate; while in the leaves, the sap undergoes 

 some changes, (not well understood,) by means of 

 the air and light, by which it is converted into a fluid 

 called latex. From the vessels of the under side of 

 the leaf, it descends by the vessels of the inner bark ; 

 part of it is carried inwards by the pores of the me- 

 dullary rays, and diffused through the stem, while the 

 remainder descends to the roots, and is distributed 

 through ,them. Sap is milky, gummy, saccharine 

 bitter, k.c, in various piants. 



At the end of spring a portion of the descending 

 sap, which is now transformed into a viscid glutinous 



matter called cambium, is deposited between the liber 

 and the wood, becomes organized into cells, and forma 



B new layer upon each. Snun a ftcrwa rds, the new 

 layers are pervaded by woody tubes and fibres, which 

 CQmmence at the leaves and grow downwards. — 

 "The number of layers in the bark and wood will al- 

 ways be equal." (Wood.; The outer bark of young 

 twigs seems to perform the same function as the 

 leaves ; in the cactus, staphelia, and other plants 

 which produce no leaves, the bark must perform the 

 same office as the leaves do in plants which produce 

 them. (Johnston.) 



Fig. 3. 



4 



[Fig. 3. — 3, horizontal section of an endogenous stem, ex- 

 hibiting the bundles of woody fibre, spiral vessels and duets, 

 irregularly disposed in the cellular tissue ; u a. a, cellular 

 EJ88Ue ; h, spiral vessels on inner side of dotted diets, c, f. ; 

 d, woody fibre on the exterior side ; 4, stem of three year's 

 growth ; a, pith ; e, bark ; b. e, d. successive annual lay- 

 ers ; 6, a, pith ; b, spiral vessels of the medullary sheath ; 

 c, dotted ducts ; d, woody fibre ; e, bark.] 



The endogenous stem exhibits no distinction of 

 bark, wood and pith, — and no concentric annual lay- 

 ers or grains. It is composed of cellular tissue, 

 woody fibres, spiral vessels and ducts, the same as 

 that of exogens. The cellular tissue exists equally 

 in all parts of the plant ; the rest are in bundles, im- 

 bedded in the stem : "each bundle consists of one or 

 more ducts, with spiral vessels adjoining their inner 

 side next to the centre of the stem, and woody fibres 

 on the outside, as in the exogen. 



"A new set of these bundles is formed annually, 

 or oftener, proceeding from trie leaves, and passing 

 downwards in the central parts of the stem, where 

 the cellular tissue is most abundant and soft. After 

 descending awhile in this manner, they turn outwards 

 and interlace themselves with those which were pre- 

 viously formed." — Rodgers^ Scientific Agriculture. 



Weeds in Gravel. — For more than ten years past, 

 says a recent writer, I have used salt, (but not in so- 

 lution,) for destroying and keeping down weeds in 

 my gravel walks, with perfect success, and without 

 perceiving that the application acted as a stimulant 

 to reproduction. The contrary is the case. I sow 

 the salt by hand in dry weather, and sweep it about 

 thin, and as regularly as possible. I have seldom oc- 

 casion to do this more than once in twelve months. 



Never court the favor of the rich, by flattering 

 their vanity and their vices. 



