Seeds are important in understanding relationships within the Cactaceae for there is much diversity in size, shape, color, and seed coat or testa pattern. Important comparative studies of seeds have been made by Barthlott and Voit (1979) and Stuppy (2001), with their data used by the International Cactaceae Systematics Group in its classification of the family. Seed characters are believed to be very stable, little affected by environmental factors.

The embryo within the seed is usually curved, and the nutritive tissue is perisperm, derived from diploid nucellar tissue of the ovary rather than from triploid endosperm, which is tissue derived from fertilization (a male nucleus and two female nuclei). The cotyledons are reduced in size compared to those of many other plant embryos. The hilum or point of attachment of the funiculus (the connection of the ovule to the wall of the ovary) is usually prominent in most members of the subfamily Cactoideae and is fused with the micropylar region (the opening into the ovule) into a distinct complex called the hilum-micro-pylar region (Barthlott and Hunt 1993,168). Some

Scanning electron micrograph of a seed of Sclerocactus parviflorus, showing the hilum-micropylar region, x38

Naked fruits of Mammillaria crinita

Primitive seeds of Pereskia (left) and seeds of Opuntia streptacantha (right) with bony envelope.

Arillate seed of Strombocactus disciformis, xl50

Scanning electron micrograph of a seed of Sclerocac-tus parviflorus, x27

cactus seeds are arillate or strophiolate, that is, with outgrowths involving the hilum-micropylar region.

Seeds of Pereskia are thought to retain the largest number of primitive characteristics in the family, having a relatively simple form and testa structure. The subfamily Opuntioideae has a unique seed type within the family: disklike with a bony funicular envelope surrounding the seed.

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