20 



DESCRIPTIVE BOTANY. 



PART I. 



and well-defined spaces, of hexagonal and other geo- 

 metric forms. In these cases the cellular tissue is so 

 arranged as to separate the lacunae from each other, 

 both by vertical and transverse di- , ^ A 10 

 visions (fig- 10.) ; and the whole is 

 placed round the axis of the stem 

 in a beautiful and symmetrical man- 

 ner. The stems and leaf- stalks of 

 aquatics are every where filled with 

 lacunc, and the air contained in 

 them serves the purpose of elevating 

 these parts towards the surface of the water. 



(22.) Vascular Tissue. This tissue consists of 

 tubes, which are also formed of membrane, to all ap- 

 pearance identical with that which composes the vesi- 

 cles of the cellular tissue. Some of these rubes bear a 

 close resemblance to the elongated cells already de- 

 scribed, and may certainly be considered as mere mo- 

 difications of that form of tissue ; and, indeed, all 

 tubes, whatever be their length, appear to taper off 

 at each extremity into conical and closed terminations 

 (fig. 11. a). A communication evidently subsists be- 

 tween some of these tubes, at the 

 point where they overlap each other 

 and are about to terminate, form- 

 ing an oval perforation of large di- 

 mensions. Some tubes are derived 

 from the apposition of cylindrical 

 cells, base to base (6), and the sub- 

 sequent obliteration of the terminal 

 portions of their membrane. In cer- 

 tain cases this membrane remains 

 wholly, or in part, in the form of 

 transverse septa or diaphragms, and 

 then these organs present a tissue in- 

 termediate between the cellular and 

 vascular. The true vessels, or long tubes, which more 

 strictly compose the vascular tissue, are distinguishable 

 into two kinds, between which, however, there are cer- 



