SCLEREID FORMATION IN THE SHOOT OF PSEUDOTSUGA TAXIFOLIA

THE INVESTIGATIONS of Foster (1944, 1945) aiid Bloch (1946) have indicated some of the major problems concerned in sclereid ontogeny. The intermittent differentiation of isolated idioblasts, which is characteristic of many plant organs, raises questions inot only in the field of histogenesis, but also properly in the realm of physiology. In the course of an investigation on shoot organization in Douglas fir (Sterling, 1946), the writer was able to observe the program of development of the nimerous sclereids which appear in the pith and cortex of that plant. The present study was undertaken to ascertain the details of sclereid ontogeny in a member of the Coniferales. Sclereids have been reported in the vegetative organs of various genera of conifers. Among the Pinaceae, they are noted as occurring in the leaves and shoot of Pseudotsuga (Allen, 1945), in the pith of Picea (Lewis and Dowding, 1924) and Cedrus (Bertrand, 1874), in the cortex of Cedrus and Larix (Neger and Kupka, 1920), in the cortex (L,ewis and Dowding, 1924) and pith (Busse, 1893) of Abies, and in the root and shoot of Keteleeria (Pirotta, 1890). None have been reported from the Cupressaceae, except as noted in Feustel's (1921) review of certain dissertations inaccessible to the writer. Bailey and Faull (1934) mention their occurrence in the old pith of Sequoia sempervirens. Of the other taxodes, sclereids have been found only in Sciadopitys (Chamberlain, 1935; Miahlert, 1885; Mohl, 1871; Solms-Laubach, 1871; and Thomas, 1865) and Cunninghamia (TThomas, 1865). In the Taxaceae, sclereids have been found in both Cephalotaxus (Bertrand, 1874; Rothert, 1899; Thomas, 1465) and Torreya (Bertrand, 1874) but not in Taxus. Acmopyle (Sahni, 1920), Phyllocladus (Bertrand, 1874), Podocarpus (Brooks and Stiles, 1910; Orr, 1944; Puchinger, 1923; Thomas, 1865), and Saxegothaea (Tison, 1909) of the Podocarpaceae are mentioned in the literature as possessing sclereids. Both Araucaria (Lopriore, 1905; Mahlert, 1885; Seward and Ford, 1906) and Agathis (Bertrand, 1874; Mohl, 1871; Puchinger, 1923; Solms-Laubach, 1871; Thibout, 1896; Thomas, 1865) have often been reported to have these elements, in their vegetative, as well as reproductive, organs. MtATERIALS AND METHODS.-The methods of investigation have been outlined in the writer's abovementioned paper. Although most of the material studied was sectioned, some shoots were mnacerated in Jeffrev's solution (Sass, 1940). The macerated material was then dehydrated in a solution of glycerine to which phloxine had been added, cleared. and mounted in Canada balsam. This technique was not adequate to delineate the cell walls sharply: Received for publication August 31, 1946. phloxine appears to be principally a protoplasmic stain, and the refractive index of the balsam is quite close to that of the lignified sclereid wall. The writer would recommend the use of a specific lignin stain and mounting in a medium with a low refractive index. Drawings for the present paper were made by camera lucida. SCLEREID ORIGIN AND DISTRIBUTION.-During the initial period of bud elongation, from February to Marclh, sclereids are absent from the tissues of the young slhoot. They appear first at the time of initial transverse expansion of the pith, which coincides with the formeation of cataphylls (Sterling, 1946). The sclereids are first noticeable among the elongated pith cells just below the developing zone of transverse expansion. They appear in the cortex only about the end of bud scale deposition, wlhen the cells furnished by the receptacular meristein (Korody, 1937; Sterling, 1946) begin to elongate markedly and raise the bud scales. The sclereids arise at this time among the elongating cells produced by this meristem in the cortex. Immediately after their first appearance in the regions noted above, isolated idioblasts can be seen developing much farther down in the shoot, in a very scattered distribution. These occur in both pith and cortex. Throughout the development of the new bud (while the pith expands and while the cell rows of the "receptacle" elongate in the cortex), new sclereids are formed continually amonag the older ones, increasing the density of their concentration. Although the sclereids of Douglas fir are especially abundant just below the area of transverse expansion in the pith, many are found in the cortex as well as the "medullary ray" areas between the vascular bundles at this level. According to Allen (1915) they also occur in the leaves and scattered throughout the rest of the shoot. Nowhere, however, are they present in as dense a concentration as in the regions of their first appearance. SCLEREID ARRANGEMENT.-In general, the sclereids of Pseudotsuga are isolated and randomlv distributed. No precise relationship to specific series of mother cells or position in a file of cells or relationship to specific divisions in a file could be ascertained, as in Monstera (Bloch, 1946). However, in the pith certain patterns of distribution recur occasionally which may be worthy of note. In many instances, two superposed cells of the medullary rib meristem will differentiate simultaneouslv to branehed sclereids, as shown in fig. 2. In one apex, several of these coupled superposed idioblasts occurred in the same vertical cell file, with undifferentiated parenchyma between the groups (as observed occasionally by Bloch, 1946). Likewise, a juxtaposed situation of the sclereids is sometimes encountered, as in fig. 9. In an older pith, there umay also be indiscriminately associated groups of three