Floral development, anatomy, and embryology of Collinsia heterophylla with some notes on ten other species of Collinsia and on Tonella tenella
Schrock, G.F.; Palser, B.F.
Bot Gaz 128(2): 83-104
The apex that produces the decussate leaf primor-dia also produces the "whorls" (separated close spirals) of floral primordia in the terminal inflorescence of C. heterophylla, with the only morphological change during the transformation being an increase to about twice the size. Flowers of C. heterophylla are hypogynous, sympetalous, zygomorphic, and bicarpellate, and the 4 stamens and 1 staminode are epipetalous. This sp., in common with other members of the Scrophulariaceae, has its floral organs produced in the sequence of sepals, stamens, petals, carpels, and ovules. Collinsia and Tonella, which have a distinct staminode, appear to occupy an intermediate position in a postulated series in evolutionary reduction of the androe-cium in the family. The order of appearance of provascular tissue in the floral organs is the same as the order of the initiation of those organs. Xylem lignification is discontinuous in the main bundles of leaves, bracts, and all floral organs. No lignified xylem is found at any time in the base of the stamen, staminode, and dorsal carpel bundles connecting these to the receptacular vascular tissue. The provascular and mature phloem connection is complete, however. Microsporogenesis, is similar to that in most angiosperms, and the single pollen grains are shed in the 2-celled stage. As in other members of the Scrophularia-ceae, the nuclei of tapetal cells of Collinsia divide mitotically, and the 2 nuclei may then fuse or they may divide and the 4 resulting nuclei fuse. In the essentially campylotropous, unitegmic, tenuinucellate ovules a single archesporial cell enlarges to form the megaspore mother cell. This undergoes meiosis, forming a linear tetrad of which the chalazal spore functions to produce an 8-nucleate megagametophyte. Development is thus of the Polygonum type. The megagemetophyte of C. heterophylla differs only in certain details (e.g. time of disintegra-tion of antipodals) from that in other members of the Scrophulariaceae. Embryo development is probably of the Onagrad type. Endosperm de-velopment is cellular. The centrally located cells divide to form the endosperm proper. In the mature seed all that remains of the integu-ment is the endothelium, the outer epidermis, and a few intervening cells where the seed was attached to the placenta. The endosperm at seed maturity consists of thick-walled (non-lignified) cells filled with large droplets of oil and tiny granules of protein. The close relationship among Collinsia spp. (C. heterophylla, C. concolor, C. austromontana, C. tinctoria, C. stricta, C. arvensis, C. bruceae, C. solitaria, C. verna, C. corymbosa, C. multicolor) and of Tonella tenella to Collinsia is supported by studies of their vascular anatomy, organography, and gametophyte development. No characters of internal structure can be used to distinguish among them.