THE TISSUES OF THE STKM 



49 



treated with solvents that remove the contents, and stained, the 

 structure of the perforated cellulose wall is well seen (Fig. 26). Longi- 

 tudinal sections show that the 

 tubes are partitioned at intervals 

 by transverse or oblique septa, 

 each of which bears a sieve- 

 plate that occupies its whole 

 area (Fig. 27) The cytoplasm 

 contracted in the preparation 

 appears as a thick cord which 

 widens out so as to cover the 



sieve-area, wi 



th which it is closely s pim§^-£&g{ 

 re are no nuclei in /ApaOD^ 



related. Thei 



mature sieve-tubes. In fine sec- 

 tions suitably stained the con- 

 tinuity of the cytoplasm by 

 threads traversing the pores can 

 be easily seen. But another way 

 of demonstrating the continuity between the sieve-plates. 



1 • •« „..i_i»,._- face view. 6 = phloem parenchvma 



is by treatment with sulphuric y v 



acid which destroys the cell-wall, while the more resistant cytoplasm 



retains its outline (Fig. 27, D). 



B 



Fig. 26. 



Phloem and cambium (cb) of Cucumber ( x 200). 

 c.c. = companion cells. s/=sieve-tubes cut through 



s/>=sieve-plate in sur- 

 F.O.B. 



IK'l/fili 



_ OQOOOB. 



if ; 





1 \ n 



Fig. 27. 



Sieve-tubes of Cucurbita Pepo. /l=surface view of a sieve-plate. B, C = longi- 

 tudinal sections showing segments of sieve-tubes. D = contents of a sieve-tube 

 after treatment with sulphuric acid, showing continuity through a sieve-plate. 

 s = companion cells. u = mucilaginous contents, pt =peripheral cytoplasm. c = 

 callusplate. f*=small lateral sieve-pit, with callus plate. ( x 540.) After Str.isburger. 



B.B. 



