The Xerophytic Mimicry Plants
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Downloaded from http://online.ucpress.edu/abt/article-pdf/30/3/201/23957/4442008.pdf by guest on 02 October 2021 The Xerophytic Mimicry Plants * John A. Jump, ElmhurstCollege, Elmhurst,Illinois Certain easily grown species of the family Aizoaceae provide demonstrationsof mimicry, adaptationto intense light and drought, and seed dispersal by rain, For the past fifteen years the author has The plants are easily grown in a sunny win- kept in cultivation a collection of plants of dow or under Gro-Lux tubes; they are the family Aizoaceael which is easily grown flowered readily in a greenhouse; and the and admirably suited to demonstration of seeds are as easily germinated as petunias some of the most remarkable adaptations of although they have a long juvenile period. the plant kingdom. At present the group The family is predominately found in the is not widely available commercially, al- deserts of south-west South Africa, the Karoo though an occasional specimen shows up and Kalahari being particularly rich in vari- in succulent collections at garden centers ety of species. Largely for this reason, most and on the plant counters of some of the of the available literature on the group is chain stores. Seeds in mixture and plants by English, German, and South African au- are available from at least one source in thors, the foremost semi-popular works being this country and may be obtained readily Higgins' translation of Schwantes,' "Flower- from abroad in a wide variety of species. ing Stones and Mid-Day Flowers," and the tri-lingual, "Mesembryanthema," by Brown, 'European writers tend to use Mesembryanthema- ceae rather than Aizoaceae. Tischer, Karsten, and LaBarre. 201 Mimicry brown, with shades of gray and violet being Mimicry, which is such a common phe- common. The definitive monograph of the nomenon in the animal kingdom, is quite genus by Nel lists fifty species, but addition- rare among plants and perhaps is most al ones have been subsequently discovered. classically illustrated by a few orchids that In nature most of the species grow buried are pollinated as a result of the attempted in the substrate with just the tops of the copulation by certain bees and wasps which leaves exposed, but in our local conditions they mimic. Mimicry plants of the Aizoaceae of less intense light, the majoritygrow with resemble a wide variety of rock fragments the leaf pair partially or entirely above the and pebbles in their markings, contours, and soil level. Each year a new leaf pair is coloration. They are highly succulent and formed at right angles within the old pair, presumably would be destroyed quickly as and the old pair gradually withers as it is a water source by indigenous animals if replaced. Schwantesreports that some plants they were more readily visible. The group have been determined by leaf base count Downloaded from http://online.ucpress.edu/abt/article-pdf/30/3/201/23957/4442008.pdf by guest on 02 October 2021 which is the prime example of mimicry is to be at least 95 years old. The flowers are the genus Lithops, (Fig. 3), called "stone composed of numerous staminodes and are face" in some American catalogues, and either white or yellow in color, (Fig. 4). "Hottentot's backside" by the imaginative The similarity of the flowers has made it Boers. These plants are virtually stemless necessary to base the different species pri- and consist of a single pair of leaves sep- marily on the basis of the shape and color arated by a more or less distinct cleft. They patternof the plant body. vary in color from green to rusty-red and Less varied than the foregoing genus, but J4~~~~~ ...~ .....j~~~~~~~~~~~~~~~~~~ ..... ~~ ~~~ . Fig. 1. Plant of Argyroderma sp. Fig. 2. Plant of Pleiospilos sp. with two granite pebbles in upper and lower left. Fig. 3. Four species of Lithops. Fig. 4. Flowers, leaves and capsule of Lithops kuibisensis. 202 The American Biology Teacher, March, 1968 highly convincing as mimicry plants are the haps the most intricate seed dispersal mech- granite-like species of Pleiospilos, (Fig. 2), anisms to be found in the plant kingdom, or "split rocks" as they are commonly called. and their structure has been treated in detail Some of these have several pairs of leaves by several authors. When the woody capsule on the plant at a time, but the observer is fully ripe, a process that may require a must look closely to decide which are leaves number of months, the upper surface splits and which are pieces of stone, if they have into five or more sectors which are the valve been grown among fragments of granite. lids. These remain tightly closed, (Fig. 5), Several species of these occasionally show until moistened by several drops of water. up on the counters of shops dealing in The water penetrates the capsule and is succulent plants, and they produce their absorbed by the highly hygroscopic mucilage large brilliant yellow flowers quite readily which is contained in cells of a palisade-like in a greenhouse or in a sunny window. A tissue beneath the valve lid. The swelling third genus to add variety might be the of these hygroscopic keels forces the valve Downloaded from http://online.ucpress.edu/abt/article-pdf/30/3/201/23957/4442008.pdf by guest on 02 October 2021 silver skins, Argyroderma (Fig. 1), which lids upward and backward until the seed produce a single pair of leaves like Lithops cavities are exposed (Figs. 6, 7, 8). Sub, but are perfectly smooth, rounded and sequent drops of water falling as rain with unmarked, mimicking grayish white, quartz considerable impact will cause the locules pebbles almost perfectly. Of the latter two to function as splash cups, and the seeds genera, Pheispilos bolusi P. Neli, and Ar- will be forcibly expelled to distances that gyroderma testiculare are among the best may be as much as a meter from the capsule. to obtain if available. In most capsules the seeds are attached to the capsule by long funiculi which anchor The Capsule and Seed Dispersal the seed and prevent it from a simply spill The capsules of the Aizoaciae include per- ing out of the capsule with overflowing ..e .B . e > .......................................................^ . _~~~~~~~~~~~~~~~~~~~~~~~~........... ........ .-^:............................ *' .... .,l | l I l I II~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~...... 222~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.221. ............ t; z -$ r C F.i._.e I ^ > r S:~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~..... Fig. 5, 6, 7. Progressive stages of opening of capsule of Fig. 11. Leaf top and miniature windows of Lithops ful. Glotiphyllum. viceps. Fig. 8. Side view of capsule shown in Fig. 7. Fig. 12. Miniature windows of Lithops fulviceps illuminated. Fig. 9. Leaf top and window of Lithops bella. from beneath. Fig. 10. Window of Lithops bella illuminated from beneath. The Xerophytic Mimicry Plants 203 water. Only by the relatively high impact of be buried an inch or so undergroundand a raindrop is the funiculus broken and the may receive light only through the window- seed catapulted out of the capsule. When ed leaf top which serves as a sort of "sky- the capsule dries out, the opening mechanism light."The leaf top may be a single window reverses itself and the valve lids close as in the case of Lithops optica; it may be a to their original position. Repeated opening window with variouslyshaped opaque mark- and closing may take place in response to ings (Fig. 9) as in Lithops leslei; it may alternate wetting and drying. The closing consist of numerous irregularly shaped process is typically a gradual one, but under "panes;"or it may be a series of tiny round favorable conditions the capsule may open miniaturewindows (Figs. 11 and 12) as in completely in less than a minute, and the Lithops terricolor.The window is apparently -movement of the valves is quite apparent opaque in Lithops gracilidelineatabut still macroscopically. Any brief description of lets in sufficientdiffused light to supply the these capsules is necessarily quite incom- chlorophyllof the innertissues. plete, as they vary markedly from one genus The transparencyof the window is not Downloaded from http://online.ucpress.edu/abt/article-pdf/30/3/201/23957/4442008.pdf by guest on 02 October 2021 to another, although the basic mechanics of at all obvious externally, but if the top of operation is somewhat similar. Since germi- the window is cut off and held between the nation of the seed is only possible during observer and a light source, it becomes im- the infrequent periods of wet weather in mediately apparent (Figs. 10 and 12). It -the desert, the adaptive value of this mech- is also instructiveto slice off the window top anism which insures dispersal only during or to cut the plant longitudinallyto observe a hard rain becomes obvious. the bright green chlorophyll-containingtis- Capsules are produced readily on green- sue, which fills up the center of the leaf house grown plants and when mature, may beneath the window and furtherdiffuses the be removed and stored for future demon- intensesunlight. strations whenever desired. They are usually The plants are apparently able to with- retained in good condition on the plant for stand not only intense insolation but also at least a year. The open capsule is beautiful intense heat, as the leaf temperature of in its geometric perfection and never fails Lithops pseudotruncatellahas been reported to arouse student interest. as 56?C with the surroundingsoil temper- Adaptationto Insolation ature still higher. At the same time temper- Quite a few of the genera of Aizoaceae atures well below freezing are not uncom- have windowed leaves. This is a character- mon in the habitat of the same species. istic that is found widely distributed in xerophytic members of quite unrelated fami- Cultivation lies.