THE TISSUES OF PLANTS. 



29 



sieve-ducts principally in being of less diameter, and in 

 having the markings but not the perforations of sieve- 

 disks. Both of these differences are such as might be 

 looked for in undeveloped sieve-tissue. 



Fig. 18.— Longitudinal section througli the sieve-tissue of Pumpkin- 

 stem, q. q, section of transverse sieve-plates; si, lateral sieve-plate; x, tliin 

 places in wall: I, the same seen in section ; ps, protoplasmic contents con- 

 tracted by the alcohol in which the specimens were soaked ; sp, proto- 

 plasm lifted off from the sieve-plate by contraction; si, protoplasm still 

 in contact with the sieve-plate. Magniiied 550 times. 



In the corresponding parts of the woody bundles of conifers and 

 ferns a sieve-tissue is found which differs somewhat from that de- 

 scribed above. In Comfers the sieve-disks, vchich are of irregular 



