MEGASPOROGENESIS AND DEVELOPMENT OF MEGAGAMETOPHYTE IN ULMUS

AN INVESTIGATION of megasporogenesis and the development of the, megagametophyte in various species of Ulmus seemed desirable because of the variability of development reported in the different species. Shattuck (1905) reported the Adoxa type of megagametophyte development (Schnarf, 1936) in the majority of cases in Ulmus americana L. He observed also that "in very many cases, there is further nuclear division without any indication of polarity, the nuclei being distributed promiscuously throughout the cytoplasm of the sac and all apparently alike." Leliveld (1935) concluded that the type of development of the megagametophyte in U. hollandica AMill. var. belgica Rehd., U. wilsoniana Schneid, and U. pumila L. var. pinnatoramosa Henry was in general similar to the Adoxa type "with the restriction that the number of antipodals has been augmented." Walker (1938) described the Adoxa type for Ulmus fulva Michx., but observed that "occasionally four antipodals are present and in one instance a megagametophyte of twelve nuclei was observed." D'Amato (1940) found that in 35 per cent of the ovules of U. campestris Mill. an eight-nucleate megagametophvte of the Adoxa type was formed. In the remaining 65 per cent of ovules the four megaspore nuclei assume a 1 + 3 arrangement, i.e., one nucleus remains toward the micropylar end of the ovule and the remaining three assume positions at the chalazal end. The egg apparatus is differentiated after two further divisions of the micropylar nucleus. A varying number of divisions of the chalazal nuclei occurred. Ten, twelve, or fourteen nucleate megagametophytes were occasionally formed. Ekdahl (1941) observed that the four megaspore nuclei assumed a 1 + 3 arrangement in Ulmus glabra Huds. in a manner similar to that described by D'Amato for U. campestris. Nuclear division follows, resulting in a 2 + 6 stage. The micropylar nuclei undergo one further division and the egg apparatus is differentiated. The chalazal nuclei likewise may undergo one further division although it may be suppressed in one or more of the nuclei. Eleven, thirteen, and fourteen nucleate megagametophytes were figured. MATERIALS AND METHODS.-Flower buds in various stages of development and open flowers of Ulmus americana L., U. fulva Michx., U. racemosa Thomas not Barkh, U. pumila L., and U. glabra Huds., var. cam perdownii Rehd., were collected from trees growing in the vicinity of Milwaukee and fixed in a number of standard fixing solutions. The material was imbedded in paraffin and sections 1 Received for publication August 15, 1949. The writer wishes to express her appreciation for support of the work from the Wisconsin Alumni Research Foundation. 8-20 p. in thickness were cut, mounted and stained in either Delafield's or Heidenhain's iron-alum haematoxylin, or with safranin and crystal violet. OBSERVATIONs.-A single anatropous ovule similar to that reported earlier for U. fulva (Walker, 1938) develops in each ovary in U. racemosa, U. pumila, and U. glabra camperdownii. The nucellus appears as a rounded protuberance of meristematic cells on the inner surface of the ovary wall. As the young ovule grows, it bends toward the stylar end of the ovary. The inner integument arises as an outgrowth of the epidermal layer at a level just below that of the primary sporogenous cell. Later, the outer integument arises below the inner integument and both integuments continue to grow and reach a level almost even with the apex of the nucellus by the time the four megaspores are formed. When the megagametophyte is mature, the integuments enclose the nucellus leaving only the micropyle. The single archesporial hypodermal cell is differentiated at the apex of the nucellus and functions directly as the megaspore mother cell (fig. 1). It is easily recognized by its larger size and deeper staining properties. The megaspore nucleus undergoes meiosis forming four megaspore nuclei in a linear row (fig. 2). At no time is there any indication of a cell wall between these nuclei. The formation of the four megaspore nuclei without the formation of cell walls agrees with the observations of other workers for the other species of Ulmus investigated. There are differences in mode of development beyond the four-nucleate stage. The four megaspore nuclei undergo one further division and a typical seven-celled eight nucleate megagametophyte is formed in some 60 per cent of the ovules of U. racemosa (fig. 3, 4). The development of the megagametophyte varies in the remaining ovules (40 per cent). The four megaspore nuclei which up to this time have been in a linear row now assume a different position. This may be due to the growth of the vacuoles and to the increase in size of the megagametophyte (fig. 5). One of the four megaspore nuclei remains at the micropylar end and the other three migrate to positions toward the opposite end (fig. 5). The nucleus at the micropylar end undergoes two further divisions and as a result the four daughter nuclei lie free in the cytoplasm (fig. 6, 7). Three of the micropylar nuclei are incorporated in cells to form the typical egg apparatus and the fourth is the polar nucleus (fig. 8). The three chalazal nuclei show great irregularity in respect to the nature and number of divisions. In approximately one-half of the cases observed, the chalazal (megaspore) nuclei undergo