232 ANATOMY AND PHYSIOLOGY OF PLANTS. 



vations and experiments on the effects of the soil, in 

 determining the form of roots. 



All roots that spring from seeds, in bis opinion, have a spindle- 

 sbape, if they are made to grow in soil that is easily penetrable. 



STEM. 



11. The stem is that part which supports the foliage 

 or fructification, consisting of a number of fine capillary 

 tubes, through which, during the time of growth, sap is 

 carried from the root throughout the whole plant. 



The varieties of stem have been previously mentioned in the Ele- 

 ments, from whence we shall infer, an equal variety must exist in 

 their structure. 



12. A perfect stem is said to consist of six parts ; viz. 

 the epidermis, the outer bark, the inner bark, the wood, 

 the alburnum, and the pith. 



The epidermis is a covering common to all parts of the plant ; 

 consisting sometimes of one layer, and sometimes of several, with 

 the other properties mentioned, in speaking of the epidermis as one 

 of the elementary textures. 



13. The cortex or outer bark is the second integu- 

 ment or covering of the vegetable, the texture of wliicli, 

 in its component parts, resembles that of the wood. 



Thus the bark appears constructed of cellular tissue, and of 

 vessels collected into sets, which at first are straight, and run 

 parallel to each other ; but by the subsequent augmentation of the 

 parts within, are separated at certain places, and touch only at a few 

 points, so as to form a reticulated appearance. 



14. In the annual shoot of trees, only a single ring 



