192 



PRACTICAL BOTANY. 



hypodermis. Last lies the single row of cells of the epi- 

 dermis. Draw the section and indicate by letters all the 

 tissues you have been able to make out. 



3. Boil an apical bud in Schultze's macerating mixture 

 a few minutes, dissect it with needles, and make a careful 

 study of the scalariform and spiral vessels and sieve tubes 



of the fibrovascular bundles, and 

 of the cells of the sclerenchyma 

 which are thick walled, pitted, 

 and pointed at both ends. 



Treat a portion of an older part 

 of the rhizome with Schultze's 

 macerating mixture, and make 

 similar studies of its tissues. 



The various vessels of the 

 fibrovascular bundles and the 

 elongated cells of the sclerenchy- 

 ma have been built up by the 

 breaking away of the end walls 

 and the fusion of a number of 

 cells into one. The markings on 

 their walls result from strength- 

 ening deposits of cellulose. The 

 spiral and scalariform vessels 

 which occupy the center of each 

 bundle make up the xylem or 

 wood of the bundle. These ves- 

 sels are filled only with air. It 

 is believed that the water containing dissolved nutrient 

 material from the soil finds passage to the upper portions 

 of the plant through the walls of these vessels. The 

 sieve tubes and similar vessels completely encase the 

 xylem ; they are known as the phloem or bark of 

 the bundle. Descending currents of nutrient material 

 elaborated in the leaves find passage in the phloem to 



IV. 



Fia. 116. Cells from the 

 rhizome of Pteris aqui- 

 lina. At the left is seen 

 the pointed end of a sieve 

 tube; B, two sieve tubes, 



5 1 and S 2 , cut lengthwise ; 



5 2 shows the back surface 

 of the tube covered with 

 sieve plates ; the back sur- 

 face of S 1 shows only two 

 small sieve plates, but its 

 side walls, w, w, show sec- 

 tions through sieve plates. 

 (After De Bary.) 



