112 VEGETAKLE ORGANOGRAPHY. 



Juice. Mirbel, who has called the attention of anato- 

 mists to these cavities, and has described them with care, 

 has designated them by the general name of Lacunce. 

 Rudolphi, who considers them as special organs, 

 calls them, for this reason, Vaisseaux Pneumatiques. 

 Link designates very well their origin and uses, in 

 giving them the name of Accidental Receptacles of Air. 

 Kieser gives them the name of Cellules cVAir. A long 

 time since I adopted the name of Air Cavities 

 (cavitates aereee ; cavites aeriennesj, which appears to 

 me more exact and suitable than those hitherto pro- 

 posed. 



If we examine, at its first development, the interior 

 of the stem of a grass, for instance, we remark that it is 

 full of a dilated but regular cellular tissue, continuous 

 throughout all its parts : after a certain time, and when 

 the stem increases transversely, this cellular tissue, not 

 being able to be distended beyond a certain limit, is 

 broken, and forms in the interval between each knot a 

 central tubular cavity, full of air, which appears lined 

 with a dry membrane, which is nothing but a false 

 membrane formed by the remains of the cellular tissue. 



When we examine in the same manner the pith of the 

 Walnut, we see that in its infancy it presents a regular 

 cellular tissue full of watery juice; after a little while 

 this juice is absorbed by the development of the 

 branch, the pith becomes dry, the branch elongates, 

 and the dried pith is thus broken into little transverse 

 disks, which leave between them disciform air cavities. 

 The pith of the White Jessamine (Jasminum officinale) 

 presents in the same manner very regular disks. 



The same phenomenon takes place very frequently, 

 but with less regularity, in aquatic plants, the tissue of 

 which is loose, and the growth very rapid ; we observe 

 in their stems, petioles, and peduncles, air cavities often 



