THE TISSUES OF PLANTS. 21 
sieve-ducts principally in being of less diameter, and in 
having the markings but not the perforations of sieve-discs. 
Fig. 18.—Longitudinal section through the sieve-tissue of Pumpkin-stem. q,q, 
section of transverse sieve-plates: si, lateral sieve-plate; x. thin places in wall: 
1, the same seen in section; ps, protoplasmic contents contracted by the alcoho 
in which the specimens were soaked; sp, protoplasm lifted off from the sieve- 
plate by contraction; sl, 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 somewhat from that de- 
