In the bi-collateral vascular bundle (Plate 127) the xylem is in between two groups of phloem—namely, an inner group and an outer group.

In pumpkin stem a bundle occurs in each angle of the stem. The entire bundle is surrounded by parenchyma cells.

In an individual bundle the xylem consists of large circular vessels and a phloem containing large sieve cells, many of which show the yellow porous sieve plates.

OPEN COLLATERAL VASCULAR BUNDLES

In the open collateral bundle (Plate 100) the xylem and phloem are separated by the cambium layer, which, through its divisions, causes the stem to increase in thickness each year. This type of bundle is characteristic of the stems and roots of dicotyledonous plants.

The bi-collateral bundle occurs in many leaves. The xylem in such cases is central, the phloem strands occupying upper and lower peripheral positions.

INDEX

Transcriber’s Notes

The Table of Illustrations at the beginning of the book was created by the transcriber.