42 



BOTANY OF THE LIVING PLANT 



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 obHque septa, 

 each of which bears a sieve- 

 plate that occupies its whole 

 area. The cytoplasm contracted 

 in the preparation appears as a 

 thick cord which widens out so 

 as to cover the sieve-area, with 

 which it is closely related. There 

 are no nuclei in mature sieve- 

 tubes. In fine sections suitably 

 stained the continuity of the 

 cytoplasm by threads traversing 

 the pores can be easily seen. But 

 another way of demonstrating 

 the continuity is by treatment 

 with sulphuric acid which des- 

 more resistant cytoplasm retains its 



V ^ 



■■--'? 



'')'dLSo}i 



Fig. 26. 

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

 c.c. = companion ceils. s< = sieve- tubes cut through 

 between the sieve-plates. si>= sieve-plate in sur- 

 face view. ^ = phloem parenchyma. F.O.B. 



troys the cell-wall, while the 

 outhne (Fig. 28, D). 



1.1. iij 



Fig. 28. 



Sieve-tubes of Cucurbita Pepo. /I = surf ace 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. «= mucilaginous contents. ^r=peripheral cytoplasm. 

 c=caUus plate. c"'=small lateral sieve-pit, with callus plate. ( x 540.) S. 



