SIEVE-TUBES. 1 73 



The first of these two chief forms, which may be termed that with transverse 

 or flat ends, is by far the most general, and is almost exclusively present in the 

 ' primary ' vascular bundles (Chap. VIII) ; the second, or sharp-ended form, pre- 

 ponderates equally in the secondary bast of Dicotyledons. Exceptions are however 

 found to this rule, e.g. the beautifully-bevelled members of tubes in the vascular 

 bundles of stems of Calamus, and the roots of Aroideae (e. g. Philodendron Imbe) ; 

 and on the other hand the flat-ended tubes of the secondary bast of Fagus sylvatica, 

 Quillaja saponaria, Ficus elastica, Madura, &c. 



The size of the members of the sieve-tubes varies, especially in diff"erent species, 

 no less than that of the vessels. The same rules hold for the length of their members 

 as for those of the vessels : but the maxima of diameter of the latter are not attained 

 by the widest sieve-tubes. The widest sieve-tubes attain a diameter on the average 

 not more than 0.02 »"™ to 0.08 "^i"; Cucurbita ; species of Bignonia, Phytocrene, 

 Calamus, &c. On the other hand, extremely narrow and insignificant ones are to 

 be found, especially in many, but not all, succulent plants, and in such as have latex 

 (e. g. Asclepiadaceae, Crassulaceae, &c. Comp. Chap. VIII). 



A few measurements of large members of sieve-tubes may be given below, but with 

 the remark that the measurements of length in long members are only approximately 

 made, or from single specimens, owing to the extreme difficulty of neatly isolating such 

 delicate organs. 



Length. Diameter. 



Internodes of mm. mm, 



Cucurbita Pepo 0.370 — 0450 .... 0.045 — 0050 



Lagenaria vulgaris .... 0-125 — 0-200 .... 0-025 — 0-040 



Calamus Rotang over 2™'^ .... 0-030 — 0-050 



Potamogeton natans . . . . 0275 .... to 0.025 



Bignonia spec. (Mohl) ... to 1-35 .... 0-450 



Vitis vinifera, bast .... about 0-6 



Root of Philodendron Imbe . to more than 2'°™. 



As regards the longitudinal and lateral connections of the members, and the 

 branchings which in certain cases are thus produced, the same rules apply in the 

 main as in the case of the vessels. 



The walls of the sieve-tubes are always delicate, not lignified, colourless cellulose 

 membranes. The sieve-plates characteristic of them only occur on those surfaces 

 where the members abut on similar elements. The sieve-plate is a sharply-limited 

 part of the wall, like a large shallow pit, which is originally, and often throughout 

 life, less thickened over its whole surface than the wall surrounding it. It is thickly 

 covered over its whole extent with round or polygonal secondary pits, which are 

 separated from one another by narrow bands of membrane : it thus resembles a fine 

 sieve, net, or lattice (Figs. 65-73). The sieve-plates of members of tubes which are 

 contiguous fit with all their secondary pits exactly on one another, and in them the 

 intervening wall disappears when the differentiation of tissues begins, so that holes — 

 the sieve-pores — appear, through which an open communication is established between 

 the neighbouring members. 



The original width of the sieve-pores differs according to the special case. The 

 widest occur in the Cucurbitaceae, where the largest (Cucurbita, Lagenaria) attain a 

 size of 5 /x and more : most of them are much narrower ; in the above-named Cucur- 

 bitacese only 2 /x wide ; also for the bast of Bignonia spec. Mohl states it at 2 jx, 



