M /.\ L: TE S TR UG TURK 



Cells liiive been described wliich are characterized by }ieculiar 

 markiugs on their walls. "When such cells .stand end to 

 end, the cross-partitions commonly disa])pear, with the 

 effect of forming long tubes, generally of larger diameter 

 than the other cells with which they are associated. 

 Such large cells are known as vessels, and tissue formed 

 of tiiem is called vascular or tradieary tissue. Hence 

 we have spiral, scalar if orvi, 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 Jibro-vascular systetn. 



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 (Fig. 274). Its form differs in different 

 cases. In some instances it consists of loui;- 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 b3^ the coalescence of cells 

 originally separate, but sometimes hx the continued apical 

 growth of .siniile cells. 



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 thickened and perforated. 



It may be added that single cells which resemble vessels in 

 their markings are often spoken of as tracheids {Fig. 265). 



Tissue-Systems. 



While groups of similar cells are designated tissues, we may 

 have also different combinations of these tissues in differ- 

 ent plants, or in different parts of the same pjlant, and 

 these various combinations are known as ti'i.'iue-systeins. 

 These are now usually ranged under three heads : (1) The 

 Epidermal System, including those combinations of tissue 

 which go to form the coverings of young stems, roots, and 

 leaves; (2) 'The Fihro- vascular System, including such 

 combinations as form the stringy masses which abound 

 in the substance of the higher plants ; and (0) The Fun- 

 damental Sy.stem, including the combinations of cells 

 which have undergone little or no change of form; in 

 short, all the rest of the plant except the two systems 

 first mentioned. 



The Epidermal System 



is most highly developed in Phanerogams. Fig. 27.") shows 

 a .section through the thickness of a leaf. Here it will 

 be observed that there is a closely-packed layer of cells 



Fiff. 274. 



ZDDOCDCqgC 



Fisr. 275. 



