Fig. 8.1-4a. Transverse section of vascular bundle in clover stem (Trifolium). In many dicots, phloem is difficult to examine because the cells are very narrow and there is no really obvious pattern as in monocots. Typically, the first place to look for dicot phloem is just to the exterior of xylem -- there will be a group of cells that are too small and cytoplasmic to be cortex parenchyma. Then, look for pairs of sieve tube members and companion cells: one very small, very cytoplasmic cell and one larger, empty-looking cell (three sets are indicated by pairs of arrows). There is a good chance that you have found the sieve tube members and the companion cells, but in many cases, it will be difficult to be absolutely certain. With luck, the companion cell will appear to be a sister cell to the sieve tube member, basically, it will appear to be just a corner or side of the sieve tube member that has been cut off to be a separate cell. But the three companion cells here do not have that appearance.

            Notice that this micrograph shows many cells that are so small and cytoplasmic that they could be companion cells but they do not have any neighbor that is obviously a sieve tube member. This makes phloem of many dicots difficult to study. It becomes easier with experience.