TUE PRINCIPAL TISSUES. 



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. 



Fig. 68. Longitudinal tangential section of the 



young bark of the grape (Wis tin if era), taker 

 the beginning of July. $, s, sieve tubes, with i 



Jll the form described ti n sof the transverse plates iu the left-hand sie 



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



It IS found OnlV shown ; t, m, medullary rays, with crystals in 



,1 " some of the cells between the sieve tabes them- 



One Ot the COmpO- selves, and between them and the medullary rays, 



fa n-t +1 1,1 a e masses of parenchyma iphloem purenchyma) 



the phloem X 145.-Aftcr]JeBary. 



portion of the fibro- vascular bundle. 



105. The so-called latticed cells ar3 probably to be 



