INTERCELLULAR RELATIONS OF PROTOPLASTS. 
103 
view. First of these came the investigations of Wilhelm,* 
who, although he studied but three plants, viz., Vitis vinifera, 
Lay maria vulgaris {the Gourd), and Cucurbita Pepo (Pumpkin), 
followed out in them the history of these structures with such 
care and fidelity as to place him high in the rank of sieve tube 
investigators. He more completely stated the dependence of 
the perforation on the seasons. Rnssow,f whose earlier work 
we have already referred to, has widely and critically extended 
his observations in two memoirs of much intrinsic importance; 
while another of the highest value is that of the Polish 
observer, Janczewski.j 
Sieve tubes, whether perforate or imperforate, must be 
therefore looked upon as the essential constituent of that 
part of the fibro-vasal bundle of plants which morphologists 
variously call the liber, bast, or phloem. They have been, 
however, most completely studied in the Angiospermous 
Phanerogams. 
Typically, sieve tubes are composed of elongated cylindrical 
or prismatic cells, arranged end to end in longitudinal rows 
which traverse the entire length of the plant, sometimes 
isolated amid a different tissue, sometimes collected into 
groups, bundles, or layers. On this isolation or otherwise 
of the sieve tube depend important morphological results ; 
for, when the tube is isolated amidst parenchymatous tissue, 
the perforations, where they exist, are only between the 
constituent cells of the row, the sieve plates and perforations 
are only terminal; where, on the other hand, two sieve 
tubes are in lateral contact the sieve plates and perforations 
establish communication also between these lateral rows, i.e ., 
are both terminal and lateral. One further point of morpho¬ 
logical interest may be noticed. The constituent elements 
of a single sieve tube may join end to end by approxi¬ 
mately horizontal partition walls. Then, as in Cucurbitaceas 
generally, a cross section of the part containing the tubes 
will show the whole or nearly the whole of some sieve plate 
in surface view. Such a surface view of the sieve plate of 
* Wilhelm, “ Beitrage zur Kenntniss des Siebrohrenapparates 
dicotyler Pflanzen,” Leipzig, 1880. 
f Russow, “ Verbreitung der Callusplatten bei den Gefasspflanzen,” 
Sitz. der Naturf. G-esellsch. zu Dorpat, 1881, and a recent memoir in 
the same, of date Feb., 1882, translated into Annales des Sc. Nat., 
1882, series vi., tome xiv., pp. 167—215. 
I Janczewski, “ Etudes comparees sur les tubes cribreux,” M6m. 
Soc. des Sc. Nat. de Cherbourg, xxiii., 1882, p. 209, etc., with eight 
double plates ; reprinted in condensed form in Ann. des Sc. Nat., 
ser. vi., tome xiv., pp. 50—166, with six single plates. 
