Floral Development, Anatomy, and Embryology of Collinsia heterophylla with Some Notes on Ten Other Species of Collinsia and on Tonella tenella

Abstract

The apex that produces the decussate leaf primordia also produces the "whorls" (separated close spirals) of floral primordia in the terminal inflorescence of Collinsia heterophylla, with the only morphological change during the transformation being an increase to about twice the size. A two-layered tunica surrounding a densely staining corpus occurs in vegetative, inflorescence, and floral apexes. Flowers of C. heterophylla are hypogynous, sympetalous, zygomorphic, and bicarpellate, and the four stamens and one staminode are epipetalous. This species, 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 androecium in the family. In the base of the receptacle 10 bundles (five median sepal alternating with five joint lateral sepal-petal bundles) diverge outward, leaving gaps. The staminode-dorsal carpel bundle is undiverged from the posterior median sepal bundle at origin but soon separates from it tangentially. The median sepal gaps soon close, and almost immediately the four stamen bundles diverge, leaving large gaps which gradually close. Above this level one dorsal carpel trace arises from the vascular cylinder, and the other separates tangentially (or obliquely) from the staminode bundle. Shortly after its divergence a dorsal carpel bundle gives rise to two small laterals. Slightly below the base of the ovary two pairs of lateral carpel bundles arise from the vascular cylinder before it splits to form the two ventral carpel complexes. The latter enter the axis of the ovary in the plane of the septum, and thus each is probably composed of two ventral traces from adjacent carpels. Two branches of each ventral carpel complex enter the placentae and supply the ovules. One other branch from each ventral carpel complex passes upward and outward through the septum. It divides, and the halves enter different carpels, where they merge with the dorsal carpel bundles near the top of the ovary-an unusual condition. Differentiation of provascular tissue proceeds acropetally in the midrib bundles of leaves, bracts, sepals, and petals and in the stamen, staminode, and all carpel bundles. 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. The direction of lignification is mainly acropetal in the median leaf, sepal, and dorsal carpel bundles, largely basipetal in the stamen and staminode bundles, and about equally acropetal and basipetal in the bract, lateral sepal, and ventral carpel bundles. 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 two-celled stage. As in other members of the Scrophulariaceae, the nuclei of tapetal cells of Collinsia divide mitotically, and the two nuclei may then fuse or they may divide and the four resulting nuclei fuse. The maximum growth rate of pollen tubes in styles was calculated to be approximately 2 56 mm per hour in the conditions under which the plants were grown. 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 eight-nucleate megagametophyte. Development is thus of the Polygonum type. Polar nuclei fuse before double fertilization occurs. The megagametophyte of C. heterophylla differs only in certain details-for example, time of disintegration of antipodals-from that in other members of the Scrophulariaceae. Embryo development is probably of the Onagrad type. Endosperm development is cellular. The endosperm cell nearest the chalaza early develops into a massive binucleate haustorium, and several (usually four) endosperm cells in the micropylar end form uninucleate haustoria. The centrally located cells divide to form the endosperm proper. In the mature seed all that remains of the integument is the endothelium, the outer epidermis-which consists of large cells that have thin tangential and thick (non-lignified) pitted radial walls-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 species of Collinsia (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.