SEEDLING MORPHOLOGY IN MARAH (CUCURBITACEAE) RELATED TO THE CALIFORNIAN MEDITERRANEAN CLIMATE

Studies in reproductive ecology were made in indigenous, western American plants in the genus Marah (Cucurbitaceae), with particular attention given plants of M. oreganus occurring in the Berkeley Hills near San Francisco Bay in California. These tuberous perennials produce capsular fruits on their annual aboveground shoots; the fruits dehisce in early summer, each one exposing about three large seeds with an average seed weight of 1.05 g. The embryo of a M. oreganus seed has two thick and fleshy cotyledons packed with protein granules. The embryonic axis, with shoot and root apices, is ca. 0.5-1.0 mm long, roughly ½0 or less the length of the seed. In the Berkeley Hills dispersal of the seeds is accomplished by nocturnal rodents, after which germination begins with the fall rains and cooler temperatures of November and December. Instead of a radicle emerging first from the seed at germination, the minute radicle and epicotyl are pushed or carried far out of the seed, down into the soil, by the elongating bases of the cotyledons. These cotyledon bases, or petioles, are fused, and as they elongate they form a hollow tube that bears the embryonic axis at its extreme tip. The cotyledonary petiole tube ceases elongation by January, when it may be 5-25 or more cm long in a seedling of M. oreganus. Then, from its tip, the radicle grows downward and the epicotyl upward—up the hollow petiole tube. The green shoot (epicotyl) reaches the soil surface by early March in this area, completes the first season's growth, and dries up by late May, when the arid summer season is beginning. But even before the epicotyl grows out of the petiole tube and above ground, the seedling's hypocotyl begins to enlarge, forming a tuber. The fleshy cotyledon blades remain in the seed coat below ground, and some food from the blades is transferred to the tuber that produces shoots in the following growing seasons. This pattern of germination and seedling establishment is now known for species of Marah and for a very few other dicotyledonous plants, all of them growing mainly in areas of hot and dry habitat that are generally referred to as having Mediterranean climate. This elongation of the fused hypogeal cotyledons is considered a complex adaptation in dicotyledons that helps ensure fast and successful seedling establishment in seasonally arid areas such as “Mediterranean” California.