JARS v62n4 - The Word: Meristem

The Word: Meristem
Bruce Palmer
Cutten, California

Autumn is upon us in the Northern Hemisphere. We observe the buds on our rhododendrons and realize that already we can tell which ones are likely to give rise to flowers and which will probably produce new growth next spring. The fate of the buds isn’t absolute yet, but while we weren’t looking generalized cells in layers near the surface began the process of specialization. These unspecialized cells and the tissues in which they are found are referred to as MERISTEM. The word is derived from the Greek merizien , to divide. It is not related to stem, even though many meristem tissues are found there. The presence of layers of meristem cells in flowering plants throughout their lives is one of the very few characteristics that distinguish them from animals. These cells, called in biology "totipotent" (Latin: totus , totally and potens , powerful), are found near the surface of the plant body. Meristem cells are what allow the plant to grow continuously. Animals begin life with totipotent cells in the embryo but as tissues specialize most cells are able only to reproduce new cells more or less like themselves. Thus the growth of animals, called determinate, stops at some point and the growth of plants does not (indeterminate).

Meristem tissues found at various locations are given different names. Those at growing tips of stems or roots are called "apical" (Latin: apex , a summit) meristems. Apical meristems may also be found in the axils (Greek: axilla , armpit) between leaves and stems. Lateral meristems are in two layers around stems and give rise to bark and to vascular (Latin: vasculum , a small vessel) tissues responsible for transporting water, nutrients and the products of photosynthesis and metabolism around the plant body. Intercalary (Latin: intercalare , to insert) meristems are located in special zones called nodes (Latin, nodus , a knot). Nodes give rise to leaves and are important in members of the grass family because new growth can arise from them at any time. You may have noticed that when you mow a blade (leaf) of grass it keeps growing from the base but if you accidentally mow the tip of a rhododendron leaf it doesn’t. That growth feature has allowed grasses to withstand attack from grazing animals and more recently from our lawnmowers.

The presence of meristem tissue is what allows us to prune plants to the size and shape we desire and expect them to survive. Meristems allow us to grow many plants from cuttings taken from stems and, less commonly, from leaves and roots. Rhododendron enthusiasts and other gardeners are fortunate in another way that plants have meristem tissues. A hybridizer who comes up with an especially desirable cross can provide tissue from an adult rhododendron to a lab which in turn can dissect out meristem tissues, shake the cells in them apart and eventually produce thousands of copies from one original plant. Anyone can do it, but my limited personal experience with orchid meristem culture tells me that the risk of contamination and incorrect procedures is so high that it isn’t worth it; better to send it to someone who does it all the time. Eventually, after myriad copies have been produced, we gardeners are the excited recipients of the result.

The mechanisms that trigger meristem tissues to produce flowers versus leaves are still not completely understood. We cannot know for certain yet that the large bud in the photo accompanying this article will produce flowers or which of the smaller ones might become new stems and leaves, but it is likely come spring that the meristem in the larger bud will produce a truss. We know that hormones (Greek: hormaein , to excite) are probably involved. Hormones are substances produced in minute quantities in cells in one part of an organism that have effects on cells elsewhere in that same organism. In animals hormones generally come from specialized tissues that we can detect. The problem of detecting hormones in plants is more difficult because the hormones are not produced in specialized organs or tissues but in ordinary cells in multiple locations. Some plant hormones have been isolated. Gibberelins and auxin, for example, control elongation of stems and cells, among other functions. Details are beyond this discussion but any good botany or plant propagation book will give plenty of information. A possible hormone that controls the production of flower parts from meristem tissues has been termed "florigen." Whether a single hormone actually exists and how it works if it does or whether florigen is a general term for a series of triggers is still being debated and researched. An excellent recent analysis of the subject is listed as a reference for the technically curious.

No matter what structures the buds on our rhodies become, the meristem cells will play the dominant role in producing them. So, the next time you’re out in the garden planting, shaping, propagating, hybridizing or just enjoying, raise a toast to the meristems, those well-known but mysterious groupings of unspecialized cells that allow all of our plants to grow and reproduce in such unique and fascinating ways.

Reference

Zeevart, Jan A. D. 2006. "Florigen: Coming of Age After 70 Years." In The Plant Cell , V. 18, pp. 1783-1789.

Bruce Palmer is a member of the Eureka Chapter. He was a teacher of biology at Maui Community College in the University of Hawaii System for twenty-five years.