THE PRINCIPAL TISaUES. 



79 



X04. Sieve Tissue. As found in the Angiosperms this 



tissue is made up of sieve ducts and the so-called latticed 

 cells. The former (the sieve ducts) consist of soft, not 

 lignified, colorless 

 tubes of rather wide 

 diameter, having at 

 long intervals horizon- 

 tal or obhquely placed 

 perforated septa. The 

 lateral walls are also 

 perforated in restrict- 

 ed areas, called sieve 

 discs, and through 

 these perforations and 

 those in the horizontal 

 walls the protoplasmic 

 contents of the con- 

 tiguous cells freely 

 unite (Figs. 67 and 

 68). In many plants 

 the sieve discs close up 

 in winter by a thick- 

 ening of their sub- 

 stance (Kg. 69). 



The tissue composed 

 of these ducts is gene- 

 rally loose, and more 

 or less intermingled 

 with parenchyma ; in 

 some cases even single 

 ducts run longitudin- 

 ally through the sub- 

 stance of other tissues. 

 In the form described 



-LongitudiTial tangential section of the 

 young bark of the grape ("Fiiis mni/era)^ taken in 

 the beginning nf J lily, s^s, sieve tnbes. with sec- 

 tions of the transvfi>e plates— in the left-hand sieve 

 ■L -J. • I. -II tube, at the top of the fignre a lateral plate is 



aoOVe it IS tound only showTi ; m, m. medullary rays, with crystals in 

 p ,-f some of the cells — between the sit-ve tnbes them- 



as one or the COmpo- selves, and between them and the medullary rays, 

 *. .^■« 4-« „4! XT- 11.. a€ masr-es of purenchyma (phloem parenchyma) 



nents of the nhloem x us.-After De Bary. 



the phloem 

 portion of the fibro-vascular bundle, 

 105. — The so-called latticed 



cells ara probably to be 



