Selective vital staining of companion cells of potato tuber and parsnip root with neutral red.

When staining the internal phloem region of a potato tuber with the vital stain neutral red, it was observed that files of elongated cells of narrow diameter were heavily stained and were easily distinguishable from the more isodiametric parenchyma cells, many of which did not stain with neutral red. The elongated cells were identified as companion cells by locating the adjacent sieve-tube members through counterstaining with aniline blue and reviewing under violet light. Of a number of other plants surveyed, only parsnip roots possessed companion cells exhibiting a similar slective staining. In other plants both the companion cells and the surrounding parenchyma cells usually stained. Sieve-tube members never accumulated neutral red. It was concluded that the vacuoles of the companion cells of the potato tuber were stained by the ion trap mechanism because of the color of the accumulated stain, the lack of staining when neutral red was applied in an acidic solution, and the complete destaining after soaking in dilute ammonium hydroxide.