Tlim TISSUES OF PLANTS. 



21 



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

 having the markings but not the perforations of sieve-discs. 



Fio. 13.— Longitudinal section through the sieve-tissue of Pumpkin-stem. g. g, 

 section of transverse sieve-plates: ai, lateral sieve-plate; x. thin places in wall; 

 Z, the same seen in section ; ps, protoplasmic contents contracted by the alcohol 

 in which the specimens were soaked; sn, protoplasm lifted off from the sieve- 

 plate by contraction ; si, protoplasm still in contact with the sieve-plate. Mag- 

 nified 550 times. 



Both of these differences are such as might be looked for 

 in undeveloped sieve-tissue. 



In the corresponding parts of the woody bundles of conifers and 

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



