This is not unusual at all! There is incredible diversity of forms and appearance among seeds — on the inside as well as the outside. The diagram in our textbook is generalized to show how some wild-collected seeds might look inside, but the actual seed anatomy of different wild species varies just as much as other plant characteristics do.
Our diagram shows a seed with a moderately thick seed coat, an embryo that is easily visible but takes up less than half the space inside, and food storage in the form of an endosperm that takes up more than half the space inside. Here are some ways your seeds may differ:
- Seed coats can range from paper-thin ones that remove easily, to ones that are hard, water-impermeable, and several millimeters thick — and everything in between. Their texture can also be anything from completely smooth to highly textured (appearing furrowed, wrinkled, bumpy, etc). They may also have tufts of hair or fluff, sharp prickles, wings, or other appendages, which are usually either related to how the seeds are dispersed or may be a deterrent to animal predators that could eat the seeds. They can also be almost any color or shape imaginable!
Sometimes seeds are even found inside an endocarp, which is another shell-like layer between the seed and the fruit (see Figure 1). The endocarp might be thin and easily removed. However, if the endocarp is hard or stony, it might be difficult to cut and remove seeds from inside. In this case, it might be more convenient to store and germinate the whole unit, treating the endocarp as a seed coat. However, be aware that sometimes one endocarp can contain multiple seeds!
Figure 1. An example of seed and fruit morphology where the fruit is composed of an external thin exocarp, an intermediate thick and fleshy mesocarp, and an internal woody endocarp that surrounds the seed.
- An embryo is a firm, usually white, tissue whose size can range from very tiny, underdeveloped, and difficult to see, to taking up much or all the space inside the seed as an almost fully developed seedling. Its shape can also vary from linear to spatulate, bent, or folded.
When embryos are tiny, it might be difficult to find them inside the seed, but if the endosperm tissue looks healthy and hydrated (and the seed looks fully formed), that often suggests that the seed is viable. In this case, the endosperm usually functions as food storage, so that the embryo consumes the endosperm and develops cotyledons while still inside the seed. When embryos are large, the cotyledons can function as food storage from the very beginning of embryo growth, and the endosperm might then be scarce or absent.
- Endosperm is a usually starchy tissue derived from the mother plant, which serves as a food source for the embryo while inside the seed. Usually it appears whitish in color, well hydrated, and fills up the space (if present) that the embryo does not.
- There are exceptions, such as orchid seeds and other dust seeds, which only have an undifferentiated embryo and a seed coat, with no food storage inside the seed. This is why many dust seeds require specific symbiotic fungi (mycorrhizae) to provide a food source for germinating seedlings.
- Be aware that some seeds might be empty, insect infested, or immature, and none of the structures above might be identifiable. It is common for wild species to produce empty seeds, which look perfectly formed from the outside but are literally empty inside, or only have an endosperm without the embryo. If they are infested, instead, they might be partially eaten or rotten inside, while sometimes looking fine from the outside. Finally, if seeds were collected too early, the embryo might not be fully formed and developed.
Different plant taxa are usually characterized by particular seed morphologies, so it can be very useful to know the species or family of the seed you are dealing with, to know what to expect when performing a cut test. We recommend consulting the article “The Comparative Internal Morphology of Seeds” (Martin 1946) and the book A Systematic Treatment of Fruit Types (Spjut 1994), where you can find the seed and fruit morphology of many taxa. For conifers, please consult the publication “Anatomy & Morphology of Conifer Tree Seed” (Kolotelo 1997).