DIFFERENTIATION AND PATTERN IN MONSTERA DELICIOSA. THE IDIOBLASTIC DEVELOPMENT OF THE TRICHOSCLEREIDS IN THE AIR ROOT

To THE student of both normal and abnormal cell behavior anything which can be learned about conditions under which cells differentiate (develop different properties during ontogenesis) is of theoretical and practical consequence. At the plant growing point embryonic cells in all likelihood start out as morphologically very similar and genetically identical totipotent units, although probably no two cells in the organism remain exactly alike for long; very soon cells can be distinguished which exhibit specialized combinations of various cellular properties and activities (as to growth, membrane structure, contents, and physiological character). Usually such cells differentiate in association with the formation of specific tissues in typical locations within the plant organ; sometimes, however, they are arranged in less regular fashion, namely, as individuals scattered through the ground tissue, to which Sachs applied the term "idioblasts." Among the latter those cells appear of special interest which occur as solitary, actively proliferating elements among the more static and regularly shaped elements of the fundamental tissue. Notable among these cells, which often combine developmental and structural features of bast fibers, sclerotic parenchyma cells, hairs and other forms (for classification see Tschirch, 1885; Foster, 1944), are certain actively growing, prosenchymatous and branched sclereids which ramify between elements of the ground tissue, such as the well-known astrosclereids of leaves (de Bary, 1884; Solereder, 1908; Foster, 1944). In a number of plants these cells are known to develop by proliferation of internal initials abutting on the intercellular space system as in the Nymphaeaceae and Menyanthoideae (Giirtler, 1905), the Araceae (van Tieghem, 1866), and Rhizophora (de Bary, 1884; Gfirtler, 1905); where the lacunae are large, the development of these cells reminds one of that of external hairs from an epidermal surface. The terms "internal hairs" (Meyen, 1837) and "trichoblasts" (Sachs, 1882; Giirtler, 1905) applied to these cells refers to this property; they are also frequently described, together with other kinds of idioblastic sclereids, as "spicular cells" (Solereder, 1908). Both the terms trichoblast and spicular cell are also used, however, to denote other kinds of plant cells; the present writer, therefore, proposes the use of the term trichosclereid for such forms as are described in this paper. These cells can be distinguished clearly both morphologically and developmentally from the brachysclereids which are differentiated in the hypodermal region of the cortex of Monstera air roots during later development (Bloch, 1944). 1 Received for publication February 8, 1946. Comparatively little is known about the induction and development of trichosclereids and other forms of idioblastic sclereids. Gurtler (1905) studied their development in Nymphaeaceae and other plants; Foster (1944, 1945) has recently investigated the development of sclereids in compact tissue of the petiole of Camellia 'aponica, as well as in the more highly lacunate tissue of the lamina and petiole of Trochodendron araloides. Characteristically in all cases the idioblasts developed from small initials which, however, did not occur in any definite pattern; in Trochodendron, according to Foster, all spongy parenchyma cells retain the capacity to form sclereid initials. The most outstanding feature of the sclereids in Camellia and Trochodendron appears to be their capacity to grow under different intercellular conditions. Thus in Camellia, cell processes intrude between densely arranged collenchyma or parenchyma cells and cease growth when a large space is entered, while in Trochodendron, where sclereid formation occurs over a relatively long period of time, the branches of the sclereids grow through spaces and intervening tissue alike. The potency here of considerable portions of the primary wall to grow out would appear as a property which these cells share with those of the ordinary parenchymatous type which retain the capacity for wall growth for considerable time. It appears significant that in Camellia, where the growth is almost entirely intercellular, but in close contact with the parenchyma walls, pit connections are apparently re-established between the walls of parenchyma cells and sclereids, much as in other intrusively growing fibers, while in Trochodendron pits are restricted to the central part of the sclereid which remained in contact with parenchyma cells. The present writer has worked with the long, pointed, fibrous and more or less branched cells which ramify in air spaces of various tissues of Araceae,especially the Monsteroideae. These have been described several times on account of their very peculiar and variable forms (van Tieghem, 1866; Wiesner, 1875; Lierau, 1888; Gurtler, 1905; Solereder, 1919; Solereder and Meyer, 1928), but relatively little is known about their development. Van Tieghem (1866) mentions their origin from small, parenchymatous initials which form processes into the adjacent intercellular space system. These processes may be seen readily enough in transverse sections through mature regions of the mesophyll of leaves or the cortex of roots and air roots (fig. 8); the early origin of the initials, however, requires more intensive study. The author noticed previously (1944) that in the air root of Monstera deliciosa the initials of these sclereids appear very early near the