24- DESCRIPTIVE BOTANY. PART I. 



left where the leaf falls, we may conceive the vesicles 

 which are contiguous to the plane of separation on 

 either side, to be so arranged, that all their ends lie in 

 this plane (fig. 14. c c"). 



(26.) Contents of the Tubes. A considerable diver- 

 sity of opinion exists as to the probable uses of the vas- 

 cular tissue in those plants in which it is found. Some 

 observers consider the tracheae destined to convey air 

 through various parts of the plant; and support their 

 opinion by the fact, that air is very commonly to be 

 observed in them, at least during certain seasons of the 

 year. Others consider all vessels to be channels for the 

 sap and nutritious juices. That most of them contain 

 liquid matter is sufficiently evident, but what may be 

 the precise use of each in particular is at present very 

 uncertain. 



(27.) Vital Vessels. Besides the tracheae and ducts, 

 just described, there is found in certain plants, and 

 possibly in all where the vascular tissue is most de- 

 veloped, a sort of network formed of anastomosing tubes 

 (Jig. 15.) and situate a little way beneath the surface of 

 the bark, through which fluids cer- 

 tainly pass, in a manner we shall 

 hereafter describe. These tubes are 

 termed " vital vessels," or " ducts of 

 the latex," by their discoverer, M. 

 Schultz. They are by far the smallest 

 of all the tubes, and extremely diffi- 

 cult to be detected in young shoots, 

 but may be seen with tolerable fa- 

 cility as they become older. They are 

 entirely without markings of any kind, 

 and are found in all parts of the plant, from the roots 

 to the leaves. 



(28.) Compound Organs. The organs hitherto de- 

 scribed, may be considered as the organic elements out 

 of which plants are constructed, just as we say that 

 minerals are formed out of certain integrant molecules. 

 We have next to notice the various compound organs, 



