OF VEGETABLE TISSUE. 21 



distended in such a manner as to form cavities 

 filled with air. They are sometimes composed 

 of large cells regularly arranged, in which case 

 they are essential to the species, as in water- 

 plants ; in other instances they are merely occa- 

 sioned by the distension of the cellular tissue. 



13. Articulations and Dehiscences. At cer- 

 tain parts of a plant, the cells or vessels instead 

 of being, as usual, dovetailed together, so as to 

 afford the greatest strength, are all arranged in 

 one plane, and consequently easily disunited ; at 

 these points, called articulations, all parts of 

 plants which naturally fall of, as the leaves of 

 deciduous trees for example, separate; where 

 these articulations do not exist, the parts may 

 perish, dry up, and be destroyed by degrees, but 

 are never detached entire. The surface left ex- 

 posed by the fall of the organ which was attached 

 to the plant by such an articulation, is called a 

 cicatrice or scar. Dehiscence consists in a de- 

 terminate and regular rupture, such as takes 

 place when fruits, arrived at maturity, burst open 

 (the beech-mast, for instance) ; the lines which 

 mark the direction these separations will take 

 are often rather prominent, and may be observed 

 before the ripening of the part the term suture 

 has been applied to them. 



