TISSUE. 



91 



rally, as the viscid excretion of some stems, and the nectar 

 of nectaries. 



ii6. Sieve-tubes (Fig. 209, sie) 

 are ducts, not lignified, colorless, 

 having at long intervals horizontal, 

 or oblique septa, which are perfora- 

 ted. Sometimes the lateral walls 

 are at different places perforated, 

 and these are called sieve-discs. 

 Through the perforations the proto- 

 plasmic contents of the cells freely 

 unite. In latticed cells the mark- 

 ings, but not the perforations of the 

 sieve-discs, are present. They may, 207 



therefore, be regarded as undeveloped sieve-tissue. The 

 tracheary tissue (Fig. 209) comprises vessels, or ducts, 

 with walls thickened in a spiral (Fig. 209, sp), — often modi- 

 fied so as to be ringed (Fig. 209, an), — reticulated (Fig. 

 209, ret), scalariform (Fig. 209, sea), or pitted manner. 

 The diameter is usually much greater than that of the sur- 

 rounding cells. They contain proto- 

 plasm when young, which disappears 

 later, and then they are filled with 

 air. Single closed cells, which other- 

 wise possess the characters of vessels, 

 are called tracheides. 



117. In the lowest plants (and in 

 the earliest stage in the development 

 of higher plants) the cells are all 

 alike, and the tissue is, therefore, 

 uniform throughout. But as we 



Fig. 207. Colknchyma. Tig. 208. Laliciferous vessels. 



