186 VEGETABLE ORGANOGRAPHY. 



little channels visible on the outside of several barks. 

 This increase in length of the layers tends to explain 

 several cavities, or intervals, which are formed in the 

 medullar}^ cavities of Dicotyledons ; for example, in the 

 Sunflower ( Helianthus ) , a great number of the Chico- 

 raceffi, &c. 



The formation of the new layers, both woody and 

 cortical, is, in following the observations of Dutrochet, 

 a phenomenon different from the preceding. A layer 

 of alburnum and one of bark, simply in juxta-position, 

 are formed at the same time, which at first present the 

 appearance of a simple jelly ; but this is not a simple 

 deposited juice, but a substance abeady presenting 

 traces of organization and the appearance of a young 

 tissue. The existence of this young layer is very easily 

 recognised, on examining, in spring, the roots of Dip- 

 sacus Fullonum, Eryngium campestre, &c. Each of these 

 layers distinctly presents a cellular zone, (which repre- 

 sents the pith,) and a fibrous one. The cellular zones 

 of each layer are developed the first in spring, and then 

 they are contiguous ; soon the two fibrous zones, the 

 one woolly and the other cortical, are developed between 

 them ; and thus it takes place each year. This deve- 

 lopment of new layers, which Dutrochet names the 

 increase in thickness, takes place during the whole hfe 

 of the plants ; that in width continues indefinitely in the 

 bark of trees, which always retains a certain softness ; 

 but it is earl}/ arrested in the solid parts. Herbaceous 

 plants, as Sunflowers, increase in breadth as long as they 

 live, and this causes those cavities of which we have 

 spoken. Dutrochet doubts that the addition of new 

 layers takes place universally among Dicotyledons; 

 founding his opinion upon this — that these layers are 

 not distinct in some long-lived roots, such as the Chi- 

 cory ( Chicorium Intyhus), &c. ; but it is moi'e probable 



