2 STRUCTURAL AND PHYSIOLOGICAL BOTANY. 



6. Their component parts are held together by an organic 

 mucus, out of which the tissue itself is generated. 



7. Tissue is found in the form of the cellular, the woody, the 

 vascular, the pitted, and the laticiferous, each of which has 

 certain modifications, constituting the Elementary organs. 



I. ELEMENTARY ORGANS. 



8. Of these, CELLULAR TISSUE (Tela cellulosa, Lat. ; Tissu 

 cellulaire, Fr. ; Pulp and Parenchyma, of old writers ; Zellen- 

 gewebe, Germ.) is the only form universally found in plants ; 

 the other forms are often either partially or entirely wanting. 



9. Cellular tissue is composed of vesicles, the sides of which 

 are not originally perforated by visible pores (22). 



10. Each vesicle is a distinct individual, cohering with the 

 vesicle with which it is in contact ; and originating from a 

 primitive point or cytollast 1 , which either remains visible on its 

 sides or is absorbed. 



11. Therefore the apparently simple membrane which di- 

 vides two contiguous cells is in fact double. 



12. If the adhesion of the contiguous cells be imperfect, 

 spaces will exist between them. Such spaces are called inter- 

 cellular passages. 



13. The sides of cellular tissue are often thickened by the 

 deposit, on their inner surface, of matter of lignification or 

 sclerogen z , which is stratified, and often pierced with passages 

 leading to the circumference. 



14. The cells contain fluid; grains of colouring matter 

 (cJiromule., chromogen, or chlorophyll) ; starch in granules (pe- 

 renchyma) ; and crystals, which, when acicular, are named 

 raphides. 



15. The vesicles of cellular tissue, when separate, are 

 round or oblong ; when slightly and equally pressed together, 

 they acquire a dodecahedral appearance 4 , with an hexagonal 

 section ; stretched lengthwise they become prismatical, cylin- 

 drical, fusiform, &c. 



16. When cellular tissue is composed of vesicles fitting 

 together by their plane faces, it is called in general terms 

 parenchyma ; and prosencJiyma if the vesicles are fusiform. 

 Both these are sometimes branched, and their divisions inos- 

 culate. 



