TEE 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 

 lignifled, colorless ^ |(//1 



tubes of rather wide {^j^nili 



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 

 Viralls 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- j^g es.— Iionpitudinal tangential section of the 

 stancpof ntVipr-fissiiPH young bark of the grape (Wis vinifera), taken in 

 Stance OI Otner tissues. ^^^ blglnmng of July. s,s, sieve tubes, with 8cc- 

 ]n the form described tionsof the transverse plates— in the left-hand sieve 

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



above it is found only shown ; m, m, medullary rays, with crystals in 

 . -^ some of the cells — between the sieve tubes them- 



as OnC' 01 the COmpO- selves, and between them and the medullary rayb», 

 ai e masses of parenchyma (phloem parenchyma), 

 X 145.— After ~ 



nents of the phloem X 145.— After De Bary.' 



portion of the fibro-vascular bundle. 

 105. — The so-called latticed cells ar 



probably to be 



