174 



SIEVE-TUBES. 



which would be too high an average for most plants : in rtiany Angiosperms 

 with small sieve-tubes they are certainly narrower, often being on the limit of clear 

 recognition. 



Further in the same plant, and even in plates lying close together, the width of 

 the pores is very unequal: in the large tubes of Cucurbita and Lagenaria, where exact 

 measurement is possible, the di3.meter of the pores of neighbouring, and otherwise 

 equally developed plates may differ by three times (comp. Fig. 65). In one and the 

 same plate the difference in size of the pores is usually small, if at all recognisable ! 

 according to Nageli they are, at least in Cucurbita, wider on the average at the 

 middle than at the margin of the plate. Very considerable differences in size and 

 form on the same plate are rare. (Comp. Hanstein, /. c, Taf. III. Fig. 4, Cucurbita.) 







Fig. 6s '•"'E- iS6. Fig. 67. 



Fig. 6s — 67. — Lagenaria vulgaris, mature internode of stem (37s). Fig. 65 and 66, transverse sections through one and the 

 same bundle of sieve-tubes (or vascular bundle) ; f>t wide-meshed sieve-plate, occupying the whole horizontal end of a member, 

 exposed, in surface view ; n, a similar one with narrow pores, t another, injured at one margin in cutting the section, callous, 

 the pores still slightly open ; the original cellulose sieve recognisable through the callus ; o sieve-plate covered by the contents 

 coagulated with alcohol ; d delicate parenchyma. Fig. 67, lateral view of two members of a sieve-tube attached end to end, the 

 plate callous, so as to close the pores completely, the original sieve is recognisable in the middle between the two masses of 

 . callus. 



According to the developmental data to hand, which however are not extensive 

 on this question, the above-described simple structure makes its appearance on all 

 sieve^plates on their first development. Many retain it long, or even throughout 

 life ; others alter, by assuming the condition termed by Hanstein callous ^. The change 

 consists in the thickening of the bands of membrane in all directions. They swell 

 perpendicular to the surface to three or more times the original thickness, and become 

 convex on their inner side : in the direction of the surface they expand so that the 

 original pores are contracted to narrow cylindrical canals, which widen out like 

 funnels only between the convexities of the inner surface. The single bands of 

 'membrane of one plate often take a different share in the callous 'thickening: this 

 increases or decreases gradually on one plate from the middle towards the edge ; in 

 this matter both sides may be alike, or the reverse : the general form of the callous 

 plate may thus be biconvex, biconcave, or plano-convex, &c. Often the inequalities 

 of thickening are irregularly distributed over one face. The callous thickening may 

 lastly extend in the direction of the surface so as to close the canals completely. Sieve- 

 plates may often be found covered with a thick mass of callus, which is not perforated, 



' [On the Callus compare Russow, Botan. Zeitg. 1881, p. 723.] 



