Single-Lobe Growth-Rate Patterns in the Lichen Parmelia caperata

The growth rate of a single lobe of Parmelia caperata was measured at intervals of 11 to 53 days for one year. Measurements were made on photographic prints enlarged to 20x, using white algae-free spots on the thallus surface as reference points. Growth occurred throughout the year but was most rapid in May-June (0.05 mm/day) when enlargement of the white spots indicated expansion of the fungal component. Each major lobe produces new lobes from two or more (up to 5 or 6) marginal bulges, but at maturity the more lateral bulges atrophy and only two or three apical lobes continue growth. Most vertical growth occurs within 1 mm of the lobe tips with cessation 1.5-2.0 mm back from the tip. Horizontal growth (increase in lobe width) is not so rapid as vertical growth and ceases when the lobe reaches about 2.5 cm in width. Lichens have a well deserved reputation as slow-growing plants. There is a large body of literature on growth measurements, going back almost 100 years, that substantiates this phenomenon. The most recent trend in this field of study is to determine seasonality of growth in more detail rather than simply prove all over again that lichens do grow (Phillips, 1963; Rydzak, 1966). European lichenologists, however, have been handicapped by long cold winters which are unfavorable for growth, and annual rates of radial increase are in general less than 3 mm per year for foliose species (Hakulinen, 1966). This same difficulty is met in northern United States where increases of less than 2 mm per year are found in the common foliose species (Hale, 1959; Brodo, 1965). In this instance one has little chance of detecting growth variation, say, in a week, for the amount of growth would not be significantly greater than the error involved in taking the measurement. The solution to this problem is obviously to find an area where lichens grow faster. Phillips (1963, 1969), for example, has been studying Parmelia conspersa and other foliose lichens in Tennessee, a region with mild winters, and has found growth rates high enough to show weekly variation. Total radial growth is 5-8 mm per year. Lichens in the vicinity of Washington, D.C., also grow quite rapidly with monthly rates as high as 1.5 mm. I have been measuring a number of quadrats, mainly of P. caperata and P. conspersa, for the past four years with a photographic technique, using prints enlarged to about 2x. At this magnification the smallest measurable increment (here considered the level of accuracy) is about 0.1 mm. Data on these quadrats and correlations with climatic factors over a four-year period are now being analyzed and will form the subject of another article. 1 This research has been supported in part by a research grant from the Washington Biologists' Field Club. 2 Department of Botany, Smithsonian Institution, Washington, D.C. 20560. This content downloaded from 40.77.167.32 on Sun, 31 Jul 2016 05:38:20 UTC All use subject to http://about.jstor.org/terms 1970] HALE: SINGLE-LOBE GROWTH RATES IN PARMELIA 73 FIGURE 1. The colony of Parmelia caperata selected for study. The lobe measured is indicated by arrows below the rule in millimeters. Photograph taken 27 April 1968. The purpose of this study was to investigate in greater detail and with high magnification the patterns of growth in a single lobe, something that has apparently never been done before. We need to know more about growth mechanisms, the extent of vertical, horizontal, and intercalary growth, and branching patterns in the thallus. These questions had to be answered before full confidence could be placed in the validity of growth measurements made from photographs enlarged two times. The lichen being investigated, Parmelia caperata (L.) Ach., grows abundantly on granite outcrops in the open oak-hickory forests on Plummers Island, Maryland, about 10 miles (16 km) west-northwest of Washington, D.C., in the Potomac River. This lichen does not grow in direct sun but prefers partially shaded rocks under the broken forest canopy. The average mean temperature for this region is about 370F (2.80C) in January and 780F (25.60C) in July. Annual rainfall totals about 40 inches (1016 mm) and falls throughout the year. Snow is infrequent and persists on the ground only a few days. The particular colony selected in this study (Fig. 1), about 6 cm in diameter, grows on a nearly level outcropping. Individual lobes were photographed with a Nikon F camera equipped with bellows attachment. At this magnification (3x on the 35-mm negative) a print enlarged to 8x10 inches gave a final magnification of about 20x. This content downloaded from 40.77.167.32 on Sun, 31 Jul 2016 05:38:20 UTC All use subject to http://about.jstor.org/terms 74 THE BRYOLOGIST [Volume 73