THE PRINCIPAL TISSUES. 



81 



Echinocystis, Ecbalium, Vitis, Bignonia, and Calamus Rotang ; also 

 Abies pectinata, Larix, Juniperus, Sequoia, and Oinkgo ; also Pteris, 

 Osmunda, Equisetum, and Lycopodium. 



(&) By making repeated horizontal sections the horizontal sieve discs 

 may be found and studied. 



(c) Alcoholic specimens afford much more satisfactory results than 

 fresh ones ; especially is this the case with the more succulent plants. 



Pig. 71. Sieve tissue of Pteris aquUina. A, end of a sieve tube isolated by macer- 

 ation : B, portiona of two tubes seen in vertical section ; in ' the sieve plates are 

 seen in front view ; at c, c, they are seen in section ; the tube s 2 has sieve plates 

 on its right and left walls, but none on its further wall, which is in contact with pa- 

 renchyma-cells ; two of the latter are seen to have nuclei in them, x 375. After De 

 Bary. 



107. Tracheary Tissue. Under this head are to be 

 grouped those vessels which, while differing considerably in 

 the details, agree in having thickened walls, which are perfo- 

 rated at the places where similar vessels touch each other. The 



