Ch. Ill, 4] AXATO:\IY OF LEAVES 31 



the smaller, and very thin on the ultimate veinlets ; and their 

 function appears to be mainly that of conducting sugar from 

 the leaf into the stem, Second, within this sheath, towards 

 the lower side, occur many small, angular, thin-walled cells 

 with protoplasmic linings, which, seen lengthwise, are found 

 greatly elongated and crossed here and there by chstinctive 

 perforated plates (Fig. 106), though in the veinlets they are 

 much simpler in structure (Fig. 9). These are the .sieve- 

 tubes and associated cells, and their function is principally 

 that of conducting the proteins made in the leaves to the stem. 



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Fig. 9. — A leaf veinlet, in longitudinal section, of Fuchsia glohosa ; 

 greatly magnified. Above are the tracheids, and below are sieve tubes and 

 associated cells, but the sheath cells do not show in the drawing, (From 

 Haberlandt's Physiological Plant Anatomy.) 



Third, just above the sieve-tubes he a number of somewhat 

 larger, angular, thick-walled cells, lacking a protoplasmic 

 lining; they are found, when seen length-nise, to run to- 

 gether into tubes, which are distinguished by characteristic 

 spiral and other markings (Fig. 101), though in the veinlets 

 thejr are onl}' spirally marked elongated cells (Fig. 9). 

 The function of these tubes and cells, called respectiveh' 

 DUCTS and tracheids, is the conduction of water from the 

 stem to all parts of the leaf. Ducts and sieve-tubes, the 

 former always above and the latter below, in conjunction 

 with the sheath cells, make up the veins, which when large 

 contain many of all three kinds, but when smaller progres- 

 sively fewer, until finally the ultimate veinlets may consist 

 of no more than the equivalent of a single duct and a .sieve- 

 tube. 



Although every chlorenchjTQa cell performs photosj-n- 



