How Do Seeds Germinate? Monocots vs. Dicots
What is the Difference Between Monocots and Dicots?
As you undoubtedly know, flowering plants are frequently separated into two different classes: the dicots and monocots. Apart from some superficial differences, such as in leaf venation and floral arrangement, the primary distinction between these two groups lies within their seed structure. All seeds have a certain number of cotyledons, or seed leaves, which end up serving a variety of functions depending on the plant species. All angiosperms have either one or two of these cotyledons (hence the terms monocot and dicot) and this difference ends up playing a big role in how the process of seed germination plays out.
First, however, I will give a brief overview of the parts of a seed and seed germination in the most general sense.
A Brief Overview of Seed Structure and Germination
As a whole seeds basically have two main parts: the embryo and everything else. Of all of these parts, the embryo is the most important as it is what will later develop into the mature plant. The rest of the seed serves to protect and nourish this embryo.
In angiosperms, the embryo has three main parts: the hypocotyl, the radical, and the cotyledon(s). The hypocotyl contains the apical meristem, so it is from this point where the above-ground stem grows from. The radical is at the base of the seed and it develops into the plant's roots. The cotyledons, like I mentioned earlier, perform different functions depending on the plant type.
So, the main difference between these two types of seeds is the purpose of the cotyledons.
In dicotyledonous plants, the cotyledons are well developed and absorb and store nutritive tissue from the endosperm of the seed. These two cotyledons are then often pushed up out of the soil (as shown in the picture of the beans at the top of this page) and serve as the plants first vegetative "leaves." This does not occur in all dicots, however. In others the cotyledons remain underground and merely provide nutrition for the growing meristems.
In monocots, the seed's endosperm is often much larger (as evident in the diagram of a wheat seed on the right) than that of the dicots. In this case, the single cotyledon stays below ground and within the seed where it digests the endosperm and transfers the stored energy to the developing embryo. Additionally, in monocots the radical portion of the embryo is aborted and only fibrous, adventitious roots are produced.
How to Spot the Difference
The best way to differentiate between monocots and dicots is to perform a seed dissection and observe the growth process of a germinated seed. If this isn't feasible, the next best thing to do is to observe some of the characteristics of a mature specimen. Monocots typically have their floral parts in numbers that are divisible by 3 (ie. 3 sepals, 3 or 6 petals, 6 stamens, etc.) and dicots have their parts generally in groups of 4,5, or more. Additionally, monocot leaves usually have parallel venation, while dicot leaves are generally more reticulated. Of course, these are guidelines rather rules and there are always exceptions!