DEVELOPMENT OF VESSELS. 



19 



and Schleiden maintain, that at a very early period the currents of 

 latex may be seen in the intercellular canals, and that ultimately a 



66 67 68 



separate membrane is developed to form the vessels. The tissue can 

 be easily examined hi the India-rubber tree, in Dandelion, Lettuce, 

 and Celandine, and in various species of Ficus and Euphorbia. 



2. DEVELOPMENT AND FUNCTIONS OF VESSELS. 



39. The simple cell is the state in which vegetable tissue first 

 makes its appearance. It is the primary form of all the textures sub- 

 sequently produced in vascular plants. To the elongation of cells, and 

 the deposition of thickening layers and fibres hi their interior, the 

 various vessels owe their origin. Thus when cells are developed as 

 continuous branching tubes, which anastomose freely, Cinenchyma is 

 formed; when they are elongated, as spindle-shaped tubes, and their 

 walls are thickened and hardened by depositions of ligneous matter, 

 they give rise to Pleurenchyma; and when elongated membranous tubes 

 are strengthened by spiral fibres, the different kinds of Fibro-vascular 

 tissue are produced. The spiral vessel may be considered as the type 

 of the last-mentioned tissue, and all its varieties may be traced to 

 changes taking place in the development of the fibre. The coil may 

 be broken hi consequence of the fibre adhering to the membrane, and 

 the latter increasing rapidly hi growth ; or the fibre may be deposited 

 irregularly, in consequence of interruptions hi growth. This view of 

 the formation of vessels is confirmed by finding hi the same tube a com- 



Fig. 66. Laticiferous vessels (Cimnchyma) from Euphorbia dulcis. 

 Figs. 67, 68. Vessels of Latex from Celandine (Chelidonium nwjus). 



