DEVELOPMENTAL POTENCY, DIFFERENTIATION AND PATTERN IN MERISTEMS OF MONSTERA DELICIOSA

INDUCTION OF regeneration processes is an important tool in the analysis of histological differentiation. Not only have terminal meristems generally the capability of more or less direct reproduction of cells in injured regions, but it is frequently possible to induce by means of the wounding technique redifferentiation of specific and varied tissue elements characteristic of later stages of tissue development in areas commonly considered as mature and stably differentiated. For the present investigation Monstera deliciosa was used, since it proved to be especially favorable material for such studies. Cellular differentiation and the development of certain tissue patterns associated with external and internal wound surfaces and centers of irritation were investigated and compared with similar structures formed at surfaces under ordinary conditions and in other members of the Araceae. Considerable capacity for resumption of meristematic activity and differentiation in wound meristems of Araceae has been reported. The reactivity and developmental potency of cells in very young tissues are strikingly demonstrated by the manner in which the natural holes and sinuses are repaired in leaves of Monstera deliciosa (Schwarz, 1878; Haberlandt, 1914). The very young leaves, ca. 8 mm. long, are still surrounded by the sheath; at this stage small areas of the lamina degenerate and die; around the margin of these necrotic areas the mesophyll cells then divide and adapt themselves to the new conditions by forming typical thick epidermal walls, whose layers exactly correspond structurally and chemically to those of the original epidermis. The author could confirm these observations and only occasionally noted slight irregularities in cell shape, in places where the "secondary" epidermis made contact with the original one (fig. 11). Redifferentiation of specialized cells is not limited to such very young tissues. For example, in various Philodendrons there is normally present a characteristic surface tissue in the differentiated zone of roots, consisting of one or several cell layers of epidermal, and one of exodermal character; toward the inside there follow one or several layers of hypodermal sclereides and fundamental cortical parenchvma (Lierau, 1888; Solereder-Meyer, 1928). In some species of Araceae this outer zone of lignified sclereides is "primarily" differentiated from cells originally present below the exodermis; in other species it is later formed from ground tissue in conjunction with "secondary" or rather revival of meristematic or metabolic activities, under conditions of normal cork formation or, as is shown here, wound healing and regeneration. If air roots of Philodendron Glaziovii are wounded in differentiated regions as 1 Received for publication September 13, 1943. deep as the cortex, a wound meristem is formed, the outer layers of which differentiate into sclereides structurally identical with those in the original hypodermis (Bloch, 1926). Both the original and the wound sclereides become connected and thus form a continuous hypodermis, comparable to the process of epidermal differentiation in the leaves discussed above (fig. 1). Some investigators have made detailed study of cork formation, especially in Monstera deliciosa; none of them has discussed the significant similarity between the "secondary" cork tissue here and the original "primary" tissue pattern in Philodendron. Schoute (1902), Giovannozzi (1911) and Philipp (1923) investigated normal cork formation in the stem. All emphasized that in later stages the cork was composed of both thin-walled cork cells and lignified sclerotic cells (brachysclereides). Schoute was interested in this case because he believed it to be a noteworthy instance in a monocotvledon of cork formation from a single initial layer, through periodic division and differentiation in the derivatives of this layer. Philipp, on the other hand, emphasized that the mode of cork formation is here not different from the well-known one of storied cork in other monocotyledons: both in stem and adventitious root several layers of cortex cells divide more or less simultaneously. In the present paper emphasis is laid on the fact that the features of both "primary" and "secondary" structure, as expressed in the differentiation of the external sclereides, are essentially alike. Both patterns are topographically related and would appear to fall morphologically and physiologically under the category of "differentiation beneath external or internal surfaces," for which numerous examples have previously been furnished (Bertrand, 1884; Bloch, 1935, 1941). This concept is supported by the types of regenerative structures obtained in the present experiments. RESULTS.-Leaves.--Leaves of different age were wounded. In every case protective tissue was formed from the exposed mesophyll cells, but its character depended on the age of the leaves, and the reactivity and potency of its cells. Very 'young leaves were injured on July 9 by means of cuts through the protective sheath of the petiole. On September 17, in various wounds at the center and margin of the lamina, no regeneration of the epidermis was found, but beneath a zone of outer collapsed, dead mesophyll cells a periderm with suberized and often some-what thickened walls had been formed. In the inner region of the scar tissue, transformation of parenchymatous elements into lignified sclereides was noted. These reactions were most vigorous in the thicker parts of the lamina. In older, already fully expanded leaves formation of sclereides was even more prominent. After seven-