54 



ON THE PLANT-CELL. 



2. Receptacles of air, formed by the destruction of a mass of 

 parenchyma. They are : 



a. Air Canals. These are formed by a portion of parenchyma 

 becoming changed first into spongiform cellular tissue, which 

 is then torn and resorbed. The walls of these canals are per- 

 fectly smooth, and the cavity is divided into 

 definite spaces by a layer of stellate cells, as 

 though interrupted by horizontal layers. Seen 

 in Canna, Nymphaa, &c. (figs. 43, 44, 45.). 



43 



/3. Air Cavities. In these a portion of parenchyma is inor- 

 dinately torn by the growth of a portion of the plant. Their walls 

 are rough with the remains of torn cells. Seen in the stems of 

 grasses, many Composites, and in the Umbelliferce. 



25. C. VESSELS. ( Vasa, Trachea.) When a row of 

 lengthened parenchyma-cells have, through resorption, their ca- 

 vities brought into continuous communication, such a series of 

 cells are called, by an unfortunate expression, a vessel ; and it 

 is distinguished from the above tissues by different names, accord- 

 ing to the nature of its walls, as spiral vessel, annular duct, porous 

 vessel, &c. (vasa spiralia, annulata, porosa). 



The nature of the vegetable vessel has been much misunderstood, from 

 the neglect of the history of its development. Various views have been 

 and are entertained ; yet nothing is easier, in the larger and more fully 

 developed parts of plants, than to observe the formation of a vessel out of 

 a row of cells. In many cases, however, it may be difficult, as the forma- 

 tion of the vessel occurs at too early a period in their history to be seen. 

 In other cases, a union of the two vessels, side to side, may prevent an 

 unskilful observer from detecting what is going on in them. Nowhere is 

 the formation of vessels out of cells, and the connexion of the spiral 

 formation with this process, more easy to be observed than in the com- 

 mon balsam (Balsamina)* 



* Anatomic und Physiologic der Caeteen. 



43 Stellate cellular tissue, from the horizontal layers in the air-canals of the petiole of 

 Aponogeton distachyon. The three-cornered intercellular passages are very large, and 

 the rays of the cells proportionately long. 



44 The same. The three-cornered intercellular passages are somewhat rounded, and 

 rather small ; the rays of the cell proportionately short. The walls of the cells, between 

 two rays, are somewhat thickened. 



45 The same from the leaves of Pilularia globulifera. The cells are somewhat length- 

 ened, with short and broad rays; the parts of the wall in contact thickened ; the inter- 

 cellular passages irregularly rounded. 



