It emerges from the Talmud (Shabbat 4a) that one who places bread in an oven to bake on Shabbat can avoid a transgression by removing the bread before it bakes (although a rabbinical prohibition would nonetheless exist against doing that). This creates a question as to the nature of the prohibition of “zorea” (planting) on Shabbat. It is understood that a seed is not planted until it takes root. However, that only happens several days after the seeds are put into the ground (Rosh HaShanah 10b, Pesachim 25a). Thus, the melakhah will never be completed while it is still Shabbat, calling into question the viability of the melakhah.

There are at least two basic approaches to this question:

a) The Rashash (Shabbat 73b) asserts that baking and planting operate on the same structure. In both cases, one incurs an obligation immediately upon beginning the process. However, this is contingent on the process reaching completion; if it does not, there is no obligation at all. Thus, one who removes the bread from the oven, or removes the seeds from the ground, avoids a transgression. Similarly, if one places bread in an oven close to the end of Shabbat, he commits a transgression, even though the action will be completed after Shabbat (as is always the case as far as planting is concerned.) [The Sefat Emet (Shabbat 4a) maintains this last point on the strength of the Yerushalmi, Shabbat 5:6.]

b) The Minchat Chinukh (298) understands that baking and planting are distinct in structure. The nature of baking is such that one does not consider himself finished until the bread is ready. (See also Mishmeret Chaim, and Resp. Maharshag, II, 109.) Planting, however, is considered finished by the one doing it once he places the seeds in the ground. [See also Afikei Yam, II, 4:1, who claims that the nature of planting makes this the only viable explanation.] While it is true that the Talmud has established that a seed is not planted until it takes root, several days later, a distinction must be made between “planted” (as far as the seed is concerned) and the act of “planting” which happens earlier. Thus, while baking can be interrupted in mid-process, planting cannot. Accordingly, if one places bread in an oven close to the end of Shabbat, he does not commits a transgression, if the action will only be completed after Shabbat (See also Resp. Halakhot Ketanot,I, 266). The Shvitat HaShabbat (Zorea 13) prefers this view, but acknowledges that in consideration of the other view, one who plants on Shabbat would be well advised to remove the seeds before they take root, if that can be done without further violating Shabbat. R. Tzvi Pesach Frank, (Resp. Har Tzvi, O.C., I, 138) asserts that differentiating between planting and baking is not inherently problematic as we have no principle mandating consistency between different melakhot. [See also Resp. Minchat Shlomo, III, 107:3.]

On a related note, R. Natan Gestetner (Resp. L’Horot Natan, VII, 21) inquires in generaL, in the event of a melakhah that is completed on its own after being initiated earlier, at what point the liability is incurred, and notes that this would have ramifications in the event of one who begins such a melakhah and then becomes mentally incapacitated or dies. [See also Kovetz Shiurim, II, 1.]