170 ELEMENTS OP STBUCTUEAL BOTANY. 



are associated. Such large cells are known as vessels, 

 and tissue formed of them is called vascular or tracheary 

 tissuB. Hence we have spiral, scalariform, annular, 

 reticulated, and dotted vessels. These different kinds of 

 vessels are usually found associated with fibrous tissue, 

 and the combination of the two is known as the fibro- 

 vaseular system. 



288. Many plants, such as Dandelion, Blood-root, 

 Milkweed, and Spurge, emit a coloured or milky juice 

 when wounded. This juice is technically called the latex. 

 It is contained in a special tissue which is peculiar to 

 such plants, known as laticiferous tissue. Its form 

 differs in different cases. In some instances it consists 

 of long tubes which may or may not branch. In others, 

 the cells composing it form a net-work. As in the case 

 of vessels, the latex tubes are commonly formed by the 

 coalescence of cells originally separate, but sometimes by 

 the continued apical growth of single cells. 



289. Sieve-tissue has been already noticed. The 

 cells are usually rather wide, and the walls are not 

 hardened, but the cross-partitions between the cells are 

 thidiened and perforated. 



290. It may be added that single cells which resemble 

 vessels in their markings are often spoken of as tracheids. 



291. Tissue-Systems. While groups of similar 

 cells are designated tissues, we may have also different 

 combinations of these tissues in different plants, or in 

 different parts of the same plant, and these various 

 combinations are known as tissue-systems. These are now 

 Usually ranged under three heads : (1) The Epidermal 

 System, including those combinations of tissue which go to 



