|MadSci Network: Botany|
The question of how to define species is a tricky one, and has occupied biologists for quite a long time. While it is a useful concept, the reality is that there is no one single definition of the term “species” that will give an unambiguous answer in all cases. There are several definitions, and the boundary between two closely related species is often indistinct. Perhaps the biggest question with respect to the term is how to define bacterial species (or how to define it for any asexually reproducing microbe). Bacteria and many other single celled organisms reproduce asexually, so the “standard definition” that you point out is meaningless in this case. Even many plants reproduce only asexually, presenting another obstacle for that definition. In any case, there is a LOT of information and debate about the definition of species, and I will not attempt to summarize it in this answer. The Wikipedia article on species will give you an idea of how complex the issue is, and is a reasonable starting point if you would like to do more reading on the subject: http://en.wikipedia.org/wiki/Species
To the main question: plant breeding is in fact a hobby of sorts for many people, and garden catalogs often carry plants that are the result of interspecies crosses. Much more common in the catalogs are “hybrid” plants that are crosses of two different varieties of the same species. For example, all tomatoes are considered the same species, and there are many varieties that “breed true”. That means if the plant pollinates itself, the resulting offspring will all be phenotypically indistinguishable from the parent and from one another. When two true breeding varieties are crossed to one another, the resulting seeds are called “hybrid”, and usually give rise to seedlings that are distinct from both parents, but are phenotypically consistent (ie, all the seedling will be very similar to one another. However, seedlings derived from these hybrids will typically show a lot of variation.
Less commonly found are seeds or plants derived from crossing two different species. The latin name for such plants has an “X” in between the genus and species. Typically both parents came from the same genus, so that name remains, but a new species name is given. For example, the plant Elaeagnus x ebbingei is a hybrid of Elaeagnus macrophylla and Elaeagnus pungens. Another is Loganberry (Rubus × loganobaccus), which is a cross of blackberry and raspberry.
Triticale (Triticale hexaploide ) which is a cross of wheat and rye is unusual in that it is a hybrid whose parents are not in the same genus. Its genus name is a combination of those of its parents (Triticum and Secale). Since wheat and rye do not have the same number of chromosomes, a sraight hybrid of the two would be sterile, since meiosis would not proceed normally. However, Triticale is an allopolyploid, carrying two sets of rye chromosomes and four sets of wheat chromosomes. This gets around the meiosis problem, and is an example of what was alluded to in the response to your previous question.
On to more practical considerations: the more closely two plants are related, the more likely it is that they will be able to cross. I’d stick with plants that are in the same genus. In a nutshell, you need to transfer pollen from one plant (the “father”) to the flower (specifically the stigma) of another (the “mother”), wait for seeds to form, then plant those seeds. The tricky part is making sure the stigma is exposed ONLY to pollen from the donor plant you have chosen. To do this, you need to remove the anthers (the pollen bearing structures) from the mother plant, to make sure that plant does not pollinate itself. You also need to make sure the mother plant is not exposed to other pollen that’s blowing around or carried by insects. If you choose plants that cannot self pollinate, you can skip the tricky step of removing the anthers from the mother plant at the right time.
A brief description of the actual process can be found here: http://www.ext.vt.edu/departments/envirohort/factsheets2/landsnurs/jul91pr4.html
You will have to do some research and tinkering around with the plants you choose to make sure you can recognize the different parts of the flower.
If you can find the chromosome number of the plants you are considering, make sure both parents have the same number of chromosomes. If you get seed, but the resulting plants are infertile, it is likely that the two parents do not have the same number of chromosomes.
The book “Breed Your Own Vegetable Varieties” by Carol Deppe has a lot of practical information about doing this, and has a few tips on overcoming incompatibility barriers that would otherwise prevent two plants from breeding with one another.
Good luck, this could be a lot of fun!
Try the links in the MadSci Library for more information on Botany.