THE PRINCIPAL TISSUES. 



79 



104. 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 obliquely 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 (Fig. 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 

 above it is found onlv 

 one of the compo- 



o 



r 



Fig. 68. Longitudinal tangential section of the 

 young bark of the grape (Vitis vinifera), taken in 

 the beginning of July. #, ft, sieve tubes, with sec- 

 tions of the transverse plates in the left-hand sieve 

 tube, at the top of the figure a lateral plate is 

 shown ; m, m, medullary rays, with crystals in' 

 some of the cells between the 'sieve tnbes them- 



as One 01 the COmpO- selves, and between them and the medullary rayc, 

 _.-i-.4-r, ^j? ,1 -LI .. are masses of parenchyma (phloem purenchy 



nents the phloem x 145.-After Be Bary. 



portion of the fibre-vascular bundle. 

 105. The so-called latticed cells 



are probably to be 



