The Lady's Slipper Project

Science and Serendipity The Pink Lady’s Slipper Project Richard Primack What is the cost of reproduction? An important hypothesis is put to the test. 9 The study of plants can sometimes lead to dis- These fungi form extensive networks of fila- coveries of an unexpected nature. For over a ments in the soil that absorb water and nutri- decade now, I have been investigatmg the repro- ents. When an orchid seed germinates, the ductive biology of the pink lady’s slipper orchid seedling is tended by the fungus filaments, (Cypripedium acaule). When I was growing up which provide all the water, minerals, and in Boston in the 1950s and 1960s, my mother carbohydrates the seedling needs to survive. In would point out the occasional pmk lady’s slip- this early stage, the orchid seedling is just a per as a special, rare plant to be held in respect, small blob of white tissue below ground. Only and I remember feeling almost reverential after five years or so does the orchid begin to toward the exquisite flowers when I discovered produce its first green leaves above ground. As them on hikes as a teenager. Hikers in the east- the orchid increases in size over subsequent ern United States still react with excitement years, it finally becomes photosynthetically when they encounter the plant during a late self-sufficient. At this point, the orchid begins spring excursion, but although the flowers are to supply the fungi with excess carbohydrates beautiful and well worth seeking out, certainly in exchange for a continued supply of water and they are no longer rare. mineral nutrients. If the fungi had been extir- The idea that the pink lady’s slipper is endan- pated from the soil after centuries of farming, gered stems from events earlier in this century then the orchid would not be present until when the species was indeed rare. At that time the forest-and the fungi-had become firmly gardeners often dug up wild plants to transplant reestablished. in their own yards, and this was perceived as a Yet another unusual characteristic may have primary threat to the species’ survival. Media limited the lady’s slipper’s ability to increase: Its campaigns to protect wildflowers targeted the flowers rarely become fruits. The reason for this pink lady’s slipper, and the campaigns appear to peculiarity is that the flowers contain no nectar have left a lasting impression on the public. to attract pollinators. In fact, the flowers are During the last few decades, however, the mimic flowers that depend on the naivete of plant has achieved a tremendous increase in bumblebees and other large insects searching for numbers in New England. Woodland areas that new nectar sources. If bees visit several flowers once had a few scattered plants now boast doz- in a row in their search for nectar, they may ens, hundreds, or even thousands. In some areas transfer pollen from one plant to the stigma of of Boston, pink lady’s slippers have become the another plant in the process. Once this happens, most common woodland herb, forming dense the petals droop to prevent further bee msits, populations. Although it is tempting to see a the ovary swells, and over the next four months simple cause-and-effect relationship between a grape-sized capsule containing tens of thou- the preservation campaign and the new abun- sands of seeds develops. In the fall, the tiny dance of the species, the truth is much more seeds filter out of slits in the capsule and are complicated. The success of the pink lady’s slip- carried away by the seasonal breezes. Unfortu- per might also be explained by the recovery of nately for the pink lady’s slipper, there are not its previously disturbed habitat. Much of the enough naive bees in the forests of New Eng- eastern United States was cleared for agriculture land. As a result, most orchid flowers remain during the early history of our nation, and the unvisited, even in large populations. In a typical landscape has only gradually returned to forest population, only one or two percent of the flow- in the last century. It may be that the specific ers develop fruits. Yet because of the large growing conditions it requires were not present number of seeds per fruit, the populations earlier in the young forest. can increase over time even with low rates of One of the missing requirements may have fruiting. been the species of soil fungi with which the Under different circumstances, however, the orchid has a peculiar obligate relationship. orchid has the potential for prolific reproduc- Cypripedium acaule (pmk lady’s slipper orchid) rendered m pen and mk by Blanche Ames Ames (1878-1969) 10 manipulated to increase or decrease their levels of reproduction, how would the change affect the rate of growth and survival in a particular species? Because the orchid readily makes fruits when artificially pollinated, I decided to test the hypothesis using large natural populations in Massachusetts-at the Hammond Woods in Newton and the Case Estates in Weston. With the help of volunteers, experimental plants at these two Boston-area sites were hand pollinated, while other plants in the populations were left untouched as controls. Hand pollina- tion of the pink lady’s slipper orchid involves gently spreading open and bending back the petal pouch with one hand, then inserting the index finger of the other hand into the resulting gap between the pouch and stigmatic column until the finger contacts the sticky yellow pollen mass. If one does this just right, the entire pollen mass sticks to the finger in one grainy clump. At the next flower, the process is begun in the same way, but this time one must bend the petal pouch back further to expose the surface. The mass on the mdex The young orchid frmt expands rapidly following stigma pollen pollmation. The grape-sized capsule contains tens of finger can then be rubbed onto the glistening thousands of seeds. green stigma. If the pollen and the stigma are at the right stage of stickiness, the entire stigma tion. When flowers are artificially pollinated by surface will be coated with a covering of pollen. researchers and volunteers, the rate of fruit set By the spring of 1985 I was eagerly awaiting can easily increase to ninety percent. In 1984,I the flowering season to see the results of my realized that this property made the pink lady’s experiment of the previous year. To my surprise, slipper orchid an ideal subject for testing an I found no difference between the control and important but unproven hypothesis: namely, the experimental plants at the Hammond that reproduction exacts a cost from plants and Woods site, either in the number of plants flow- animals. Most biologists accept the idea that ering or in the average size of the plants. At the each organism has a finite supply of resources Case Estates a few more control plants than available for use in growth, survival, and repro- experimental plants were flowering and the duction. Thus, any individual organism that experimental plants seemed smaller, but the devotes a large portion of its resources to repro- difference was not substantial. At this point I duction will have a slower growth rate and a had a real dilemma: should I write up the results reduced probability of survival and subsequent right away, boldly announcing that there was reproduction. These reductions are collectively no cost of reproduction in the pink lady’s slipper termed the cost of reproduction. The hypothesis orchid? Or should I continue the experiment to is supported by observations that trees grow see if the cost showed up after a second repro- slowly in years when they fruit heavily, and that ductive episode? pregnant animals lose weight and suffer higher Because the two populations seemed to be mortality. However, a crucial missing element showing somewhat different patterns, I decided was experimental evidence: if individuals were to continue the experiment using a "press" 111 Hand pollmatmg requmed physical contortions of the volunteers, but nonetheless it proved to be one of the most popular aspects of the project. design. That is, every experimental plant hand- both sites. The results were slightly different, pollinated in 1984 was pollinated for a second however, due in part to a contrast in plant time in 1985 and was thus given a chance to health at the two localities. At Hammond make a second fruit. The hand pollinations were Woods, the control plants grew larger over the successful, with most experimental plants mak- course of the study, while the experimental ing a second fruit by the fall. As in the previous plants stayed the same average size. At the Case year, the control plants did not make fruit. By Estates, however, a disease turned the leaves the spring of 1986, the results proved worth the black every summer and probably killed many wait. A tremendous difference in plant size plants outright. The average size of control appeared between the experimental plants and plants at this site declined over the years, but the control plants, particularly at the Case the experimental plants declined in size more Estates, where the experimental plants had lost precipitously. Clearly, the extra cost of produc- twenty-five percent of their size in comparison ing fruits added to the stress of disease had with the control plants and far fewer of the major effects on the plants. experimental plants were in flower. The experiment allowed us to test a related The results were striking, but I decided to put hypothesis as well.

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