This mishnah continues the discussion of the previous mishnah - the prohibition of grafting vegetables onto trees. Rabbi Yehudah permitted this, but we follow the opinion of the first tanna, who forbade it.
A vegetable in a tree - sycamore and rue:
The mishnah rules that "ein notin yerakos b'toch sadan shel shikmah" - one may not plant vegetables inside the trunk of a sycamore tree, since this constitutes kilayim. So too, "ein markivin peigam" - rue is a fragrant wild herb with yellow flowers - onto a fragrant tree. Both cases are forbidden, because planting a vegetable inside a tree is considered kilayim, and as stated, this follows the opinion of the first tanna in the previous mishnah.
Planting a fig shoot in a chatzav:
One may not plant a fig shoot - a sapling from a fig tree - into a chatzav, in order "she'yehei mekaro".
What is a chatzav? It is a type of vegetable similar to an onion, whose roots descend very deep and perfectly straight into the ground. The Gemara says that Yehoshua made use of this plant when establishing the boundaries of Eretz Yisrael: even if the upper portion was cut off, one who digs below finds the roots going straight down, and in this way it is possible to determine precisely where the boundaries ran.
Several explanations have been offered for the meaning of the words "she'yehei mekaro":
From the word for coolness: the chatzav is very hot, and the fig shoot comes to cool it.
From the word korah, something that provides shade: the fig shoot casts shade over the chatzav.
Using the chatzav, which had especially strong roots, to help the fig shoot take root.
In any case, even though a person derives benefit from this, it is forbidden: the fig shoot is a tree, and the chatzav is a vegetable.
A vine shoot in a watermelon:
"Ein tochvin zemorah shel gefen" - one may not insert a vine shoot into a watermelon, "shetehei zorekes meimeha" - so that the liquid should pass from the watermelon into the vine. And the reason for the prohibition: "mipnei shehu ilan b'yarak" - he is joining a tree and a vegetable together.
A gourd seed in a mallow - a vegetable in a vegetable:
"Ein nosnin zera dela'as l'toch chalamis shetehei meshamarto" - one may not place a melon or gourd seed inside a mallow. The mallow is a type of edible vegetable, and its pods contain liquids that would provide a kind of protection for the gourd seeds and preserve them.
Here the case is "yarak b'yarak" - both are vegetables: both the gourd seed and the mallow. Nevertheless it is forbidden, since it constitutes kilayim between two different species of vegetable.
In summary: this mishnah lists four forbidden forms of grafting and planting - vegetables in the trunk of a sycamore, rue onto a fragrant tree, a fig shoot in a chatzav, and a vine shoot in a watermelon - all of which are forbidden as a vegetable in a tree or a tree in a vegetable; and finally, a gourd seed in a mallow, which is forbidden as a vegetable in a vegetable, even though both species are vegetables.