Cacti rely on seeds for reproduction. While the adult plants come in a vast array of shapes and sizes, from towering saguaros to small pincushions, the seeds share a set of general characteristics. The seeds are protected inside a fruit, which can be fleshy and bright or dry and inconspicuous, ready to be dispersed into the environment.
Physical Characteristics of Cactus Seeds
Cactus seeds are typically small, often measuring between 1 and 5 millimeters in length. They generally have a hard outer shell, called the testa, which acts as a protective layer for the embryo inside.
Mature cactus seeds range in coloration from deep black to dark brown or reddish-brown. Their shape is often asymmetrical, frequently described as kidney-shaped, helmet-shaped, or tear-drop shaped. The testa’s surface texture is another distinguishing feature; it can be smooth, but it is often roughened, pitted, or covered with small, irregular bumps.
Diversity Across Cactus Genera
While many cactus seeds share a general appearance, their size and specific features vary considerably depending on the genus.
For instance, the seeds from the Opuntia genus, which includes Prickly Pears, are relatively large and often have a distinct aril, a fleshy appendage that partially or completely covers the seed. These seeds frequently exhibit a flatter, more helmet-like shape compared to others. Conversely, the seeds of some Mammillaria species, such as the pincushion cacti, are notably tiny, sometimes appearing almost dust-like.
Surface texture often provides the clearest difference between genera, even when size is similar. Some seeds may be almost completely smooth, while others, like those from certain barrel cacti, feature a highly pitted or wrinkled exterior.
Seed Extraction and Cleaning
Cactus seeds are found embedded within the pulp of the fruit. The fruit itself can be fleshy and juicy, like the fruit of an Echinopsis or Prickly Pear, or dry and brittle, such as those from a Ferocactus or some Mammillaria.
Fleshy Fruit Extraction
For fleshy fruits, the process involves a technique called maceration, where the fruit is mashed or squeezed to separate the seeds from the surrounding pulp. The mixture is then placed in water, where the viable seeds will typically sink while the lighter pulp fragments float, allowing for separation by flotation. Rinsing the seeds through a fine mesh strainer helps remove any remaining sticky residue.
Dry Fruit Extraction
Dry seed pods, which are common in genera like Mammillaria, can often be gently crushed or shaken over a surface to release the seeds. After the seeds are freed from the fruit material, they must be spread out in a single layer to air-dry completely in a warm, shaded area. This drying process is crucial because any residual moisture or pulp can cause the seeds to spoil or become infected with pathogens.