The Seed

Seeds are found only in flowering plants and “gymnosperms”. As you have learned, a seed consists of:

(1)  An outer, protective seed coat, derived from the integument(s) of the ovule,

(2)  A store of nutritive material, remnants of the megagametophyte in gymnosperms, and the endosperm in flowering plants, and

(3)  The embryo representing the next sporophyte generation.

(A) Seeds vs. spores

In seed plants, the seed, not the spore, is the propagule for the next generation. Seeds have several advantages over spores in this role. Two advantages are particularly important.  First, the seed coat provides protection from mechanical damage, dessication, and other adverse conditions.  Second, the endosperm (or the cotyledons, as discussed below) provide nourishment for the seedling, enhancing its ability to become established.  The larger the seed, generally the more nutrient is available to the seedling. In mesic (moist) regions, larger seeds are found in species that inhabit shady areas. The extra storage material helping seedlings to grow above layers of organic litter and become big enough to obtain the sparse light under the forest canopy. Large seeds are also found in species that live in xeric (dry) habitats. Here, the extra storage material enables the seedling to produce deep roots that reach moist layers of soil far below the surface.

Seeds of most species will not germinate at any old time - most species’ seeds have a dormancy period, and dormancy must be “broken” before germination occurs. When conditions are appropriate, (and for some seeds very specific conditions or sequences of conditions are required), the seed can absorb water and the cells of the seed swell, causing the seed coat to rupture. For example, seeds of species living in areas with cold winters often require a moist cold treatment before they will germinate. In some cases, a specific photoperiod is also required. Desert species commonly have seeds that contain germination inhibitors that are leached out by heavy rains; light rains will not leach out sufficient inhibitor, preventing the seeds from germinating before sufficient rainfall has occurred for successful establishment and growth.

In California, vegetation adapted to fire consists of species whose seeds require a heat treatment or a variety of other treatments associated with fires before germination. Some of these “fire species” require specific chemicals leached out of charred wood; completely ashed wood will not break their dormancy. All of these dormancy breaking characteristics are easily interpreted as adaptive; seeds germinate under conditions where establishment will likely occur. Seeds germinating at different times usually are not able to establish and so die.