INTRODUCTION TO BOTANY. xliii 



theory. 2d. That it is produced by the developement of buds. 

 This is the theory of Du Petit Thouars. 3d. That the annual 

 formation of woody layers is owing to the cambium, which, 

 every year, forms at one and the same time, a new layer of 

 alburnum, and a new layer of liber. This opinion is pro- 

 fessed by Mirbel, and considered by Richard as the most pro- 

 bable. According to this third opinion, there is, every year, 

 formed in the trunk of dicotyledonous trees a new layer of 

 wood. This layer is produced by a part of the cambium, 

 which becomes organized and solidified. The alburnum 

 formed on the preceding year, acquires greater density, and 

 is changed into wood. But the liber undergoes no change; it 

 is only repaired and increased on its inner surface by means of 

 the cambium, which successively forms new layers. 



§.1. Growth in Height of Dicotyledonous Stems. 



At the time of germination, the radicle, (rootlet,) descends 

 into the earth, while the ascending caudex rises upwards. See 

 Plate, 12, and Explanation. From the upper part of the 

 stem proceeds a new centre of vegetation, from which arises a 

 young shoot. To this succeeds a third, which on the following 

 year is surmounted by a fourth, and so on. 



The trunk is, therefore, composed of a series of cones, 

 greatly lengthened out, whose tops are turned upwards, and 

 which are placed one above the other. The top of the 

 innermost cone terminates at the base of the second shoot, and 

 so on in succession, so that the number of woody layers cor- 

 responds with the number of years that the plant has lived, 

 only at the base of the trunk. Thus a stem of ten years old 

 will shew only nine layers of wood, when cut at the second 

 shoot, only eight at the third, and, lastly, only one, near the 

 top. 



§. 2. Growth of the Steins of Monocotyledonous Trees. 

 See Plate, 11, 12. 



In a palm, after germination, the leaves, which are usually 

 folded, become expanded, and appear under the form of a 

 circular cluster which springs from the collar of the root. On 



