Stkucttjee and Growth. 19 



the surface of the soil, and having buds (eyes) embedded in a 

 cellular substance, consisting principallj of starch. 



JBulhs, whether formed in the earth, as is generally the case, 

 or on the summit of the stem, as in the top or tree onion, are 

 simply leaf-buds inclosed in scales or concentric layers. 



III.— THE STEM. 



As soon as the root enters upon the performance of its proper 

 function, the stem begins to extend itself upward, and the pri- 

 mary leaf-bud, attracting to itself the food procured for it, and 

 a part of the nutritive matter stored up in the seed-leaves, ex- 

 pands, and the two or more parts or leaves of which it is 

 composed separate, and begin to manifest their distinctive 

 features. 



The stem is at first composed entirely of cellular tissue, 

 possessing neither strength nor tenacity ; but at the moment 

 that the first rudiment of a leaf appears upon its growing point, 

 the formation of woody matter commences. It consists of tough 

 fibers of extreme fineness, which take 'their rise in the leaves, 

 and thence pass downward through the cellular tissue and are 

 incorporated with the latter, giving it the necessary strength 

 and flexibility. In trees and shrubs these fibers combine 

 intimately with each other, and form what is properly called 

 wood; but in herbaceous and annual plants they constitute a 

 lax fibrous matter. The woody matter thus plunged, as it 

 were, into the cellular tissue, forms within the circumference 

 of the stem a tubular partition, separating it into two parts — 

 the bark and the pith. This gives us, in perennial stems, the 

 three general divisions of pith, wood, and bark. 



The pith consists entirely of cellular tissue, gorged at first 

 with the nourishing juices of the plant, but afterward becom- 

 ing empty and dry. 



The wood consists of the proper woody fiber, interwoven 

 with and bound together by thin plates or layers of cellular 

 tissue, passing horizontally across it, and forming what are 

 caUed the silver grain in maple, oak, etc. They represent the 



