Another diploid Tripsacum from Mexico.
The first diploid Tripsacum known to Mexico was discovered by the writers in 1947 at Acahuizotla, Guerrero. This diploid was corn‑like in that it had very broad leaves 12 to 15 cm. in width; thick, leafy stalks; a tall, robust growth habit and many spikes in the tassel. In fact the number of tassel branches, which ranged from 20 to 27 in the specimens that were collected for detailed study, exceeds the number found in most existing varieties of Mexican corn.
Following the discovery of this diploid in 1947, search for additional diploids was continued in that year and again in 1948. A very grass‑like form of reduced stature, extremely narrow leaves and with but one or two tassel branches was collected from a steep, rocky slope along the highway to Acapulco in the very arid Ca–ada del Zopilote 273.5 kilometers south of Mexico, D.F. When this collection was made on September 11, 1947, most of the plants hand flowered. A quantity of inmature seed was collected as well as live plant material and herbarium specimens including spikes still shedding pollen. The very youngest available inflorescences still in the bud stage were preserved for cytological study. From subsequent examination of the immature staminate spikes a single sporocyte preparation was obtained which indicated that the plants were tetraploid. Not being fully satisfied with this determination (this preparation night have been from a tetraploid sector in an otherwise diploid plant) an attempt was made to germinate some of the seed in order to obtain seedling root‑tip counts. This first attempt failed.
Returning from extensive explorations in the fall of 1948, which ranged as far north as Tepic in western Mexico and east to Vera Cruz, without discovering additional diploids except for more of the Acahuizotla type in that general locality, the 1947 collections were reexamined and another attempt was mde to germinate samples of seed that had failed to germinate in earlier trials. Tripsacum seed often germinates very poorly, but seedlings frequently can be obtained from samples that cannot be germinated in the usual manner by utilizing the embryo culture technique.
A special effort was made to embryo culture the immature seed of the 1947 Zopilote collection. These plants were suspected of being diploid at the time they were first collected. Their reduced size, their very narrow leaves and the small size of their anthers and pollen as compared with the more typical specimens of T. lanceolatum which they resembled most closely, suggested the size relationships that exist among the well‑known diploid and tetraploid forms of T dactyloides found in the United States. Five seedlings were obtained from the Zopilote seed and root tips were collected from three of them for cytological examination. All three seedlings were found to be diploids with 36 or approximately 36 chromosomes.
The discovery of a diploid Tripsacum of an extreme T. lanceolatum type in southern Mexico completes the assemblage of diploid representatives of all of the more important species complexes in the genus throughout its present range in the Western Hemisphere. Others reported previously include the diploid T. floridanum of southern Florida, the diploid of Z. dactyloides which occurs from Kansas southward to Texas, the diploid T. australe of Brazil and the Acahuizotla diploid which resenbles T. pilosum more closely than any other known species.
L. F. Randolph and