ROOT HAIR VARIATIONS IN PLANT SPECIES

THE PERSISTENCE and extent of root hair development have been reported by a number of investigators. Whitaker (1923) observed the root systems of several members of the Compositae growing under swamp conditions and found root hairs covering the entire root surface. A similar report by McDougall (1921) was given for several woodv legumes in which he stated that root hairs persist as long as the root epidermis is intact. The same observations have been made by Dittmer (1938, 1940) for several monocotyledonous plants and soy bean in which he made quantitative investigations of the roots and root hairs for a number of plants. The data were used to describe the use of such studies in determining a plant's soil binding efficiency-' Additional information concerning the intimate contact made by the small roots and root hairs in penetrating the soil interstices was reported by Kramer (1940). He stated that the capillary movement,,of water alone was not sufficient, under most circumstances, to supply a plant adequately with water and that by its tremendous rate of growth a plant was able to move into new soil areas for its water. Albrect et al. (1942) have also used these figures to point out the surface phenomena between the minute plant parts and the exchangeable ions of colloidal clay in adsorption. The present study was made to determine the size and extent of root hair growth in twenty plant families including thirty-seven species. METHODS AND MATERIALS.-Plants used in this study were taken from the field while growing under natural conditions. The selection of species was done on the basis of availability and to include representative members of various families of angiosperms. Only plants which were fully matured were collected and most of these were in flower or fruit at the time. The only exceptions were Ulmus pumila, Celtis occidentalis, Gleditsia triacanthos, Fraxinus lanceolata, and Catalpa catalpa, where the plants used were from 2-6 years old. The woody plants (mentioned in the previous sentence) and two grasses, Agropyron elongatum and Eragrostis curvula, were collected in the Soil Conservation Nurseries at Bernalillo, New Mexico. All other plants were obtained in their natural habitat or taken as weeds from cultivated fields. A minimum' of ten plants for each species was collected and studied. The soil was checked for pH in each case and ranged from 8.0-8.4. A block of soil enclosing the plant or plants was removed and placed in running water. After washing, small sections of 'the underground parts were placed in Petri plates containing water for microscopic study. MeasuremeInts were made of the hairs by means of an ocular micrometer, according to the method previously described, Dittmer (1937). Ma1 Received for publication May 29, 1948. croscopic examinations on the root systems of many of the plants here reported were made and the data recently published (1948). RESULTS.-Root hairs were found growing from the epidermal cells of all plants examined although they were not always found on all roots. Considerable variation in size of root hairs and range of distribution was noted among the different species, but within any one species the differences in size were comparatively slight. Table 1 lists the various species examined in the twenty families and has recorded the length and diameter.of the hairs in the different root categories. DIscussIoN.-Diameter of root hairs varied from 5 LA in Fraxinus lanceolata to 17 LA in Soya max. The diameter of the hairs was in no way correlated with length since F. lanceolata had hairs 370 LA long and 5 /A in diameter while those of Convolvulus arvensis were only 80 LA long and 14 LA in diameter. Si ilar measurements may be noted in the table for a number of plants. Within the same species and on the same plant slight differences may be observed in the diameters of hairs in the different root categories. Hairs arising from cells of the main roots are usually larger in diameter, by a micron or two, probably because the roots in this category are larger and possess larger epidermal cells from which the hairs arise. The diameter of the hair may indicate the size and perhaps shape of the epidermal cells from which it arises. Since there is little difference in diameter of hairs, epidermal cells giving rise to these structures may therefore be of a uniform size. Length of root hairs.-Length varies somewhat more among the different species but again within a species and on a single plant it is relatively constant. Hairs of Convolvulus arvensis averaged 80 LA in length while those of Equisetum Kansanum averaged 1500 ju in length. All other species ranged in between these extremes. Hairs of the older roots are usually somewhat longer while those of the secondary, tertiary, and quaternary categories generally have very little variation in length. The grasses in general had comparatively long root hairs with the exception of Setaria viridis, whose roots branched only into the secondary category, and whose root hairs averaged only about 100 ju in length on both root divisions. Extent of root hair development.-Hairs are not listed in some of the categories for a number of plants. This is especially true for dicotyledonous plants with extensive secondary thickening. Hairs were not found on the tap roots of these plants except at the tips and in many cases these were broken off or lost in collecting. Approximately 75-80 per cent of the surface area of the entire root system of dicotyledons exhibiting secondary thickening was covered with root hairs, although most of the hairs