SIEVE-TUBES 329 



walls ; the fact that the lateral walls also bear a considerable number 

 of pits suggests the existence of an active interchange of materials 

 between the leptome-parenchyma on the one hand, and the contiguous 

 conducting parenchyma and companion-cells on the other. Leptome- 

 parenchyma cells frequently become subdivided into two or more 

 segments by secondary transverse walls ; such septate leptome-paren- 

 chyma is connected by various intermediate stages with typical non- 

 prosenchymatous conducting parenchyma. 166 



Little is known concerning the functions of leptome-parenchyma. 

 A plausible theory, which, however, still requires confirmation, is that 

 it serves for the conduction of the more readily diffusible among the 

 protein compounds over considerable distances. Most probably this 

 tissue is also to some extent responsible for transferring protein com- 

 pounds to and from the sieve-tubes. Such leptome-parenchyma cells as 

 approximate closely to ordinary conducting parenchyma in structure, 

 may perhaps take part in the transportation of carbohydrates. 



3. Sieve-tubes and compemion-cells}^ 



Sieve-tubes are formed by the longitudinal fusion of rows of elon- 

 gated cells. The length of the individual segments which are clearly 

 distinguishable even when the sieve-tube is fully developed varies 

 within wide limits ; the largest known segments (exceeding 2 mm.) 

 occur in climbing plants, which likewise possess the widest sieve-tubes 

 ("02- "08 mm. in diam.). The partitions separating successive segments 

 from one another are as a rule strictly transverse in primary bundles, 

 whereas in the secondary leptome of Dicotyledons and Gymnosperms they 

 are very oblique. The septa bear the sieve-plates, which, from an onto- 

 genetic point of view, merely represent the peculiarly modified limiting 

 membranes of unusually large pits. Where the septum is strictly trans- 

 verse or nearly so, it becomes transformed bodily into a single sieve-plate, 

 except for a very narrow marginal zone (Figs. 133 and 134); a dis- 

 tinctly oblique septum, on the contrary, always bears several transversely 

 elongated sieve- plates, placed one above the other and separated by narrow 

 strips of imperforate membrane (Fig. 135). Sieve-plates also occur on the 

 lateral walls of the tubes ; these are variously disposed, but are always 

 restricted to the walls that abut against other sieve-tubes. 



Every sieve-plate is partially thickened in a characteristic manner, 

 the thick portions forming a delicate network. The meshes of this 

 reticulum are known as sieve-fields. Among Gymnosperms they are 

 perforated by protoplasmic threads, which later become converted into 

 so-called slime-strings. In Angiosperms, on the other hand, the regions 

 of the limiting membrane corresponding to the sieve-fields of 

 Gymnosperms become completely absorbed ; the sieve-plates are thus 



