Prohibited tree grafting consists of joining two tree species to one another in such a way that each part retains its own qualities, with no genetic merging between the two parts. By contrast, cross-pollinating two species is permitted. Every fruit-bearing plant produces flowers, and in the flower are male stamens and a female ovary. Through insects, butterflies, and wind, pollen from the male stamens reaches the ovary and the female ovules within it. Through this process, fruit is formed, containing the seed from which another tree can grow. If pollen from the male stamens is taken from a tree of one species and brought together with the female ovary of another species, a hybrid of two different species is created, and the fruit that grows will have genetic traits shared by its two “parents.” Yet each hybridization will yield a somewhat different type of fruit, because each time different components of each species’ genetic sequence are expressed, creating a new blend.
Usually, cross-pollinating two species does not succeed: pollen from the male stamens does not fertilize the ovules in the ovary, and even when fertilization occurs, the fruit produced is usually poor. Sometimes, however, the union succeeds and produces fruit with a distinctive taste, or a tree with special qualities, such as the ability to grow in a climate in which it previously could not survive. To this end, many scientists work to create numerous hybrids. Some rabbis maintain that just as one may not graft two tree species onto one another, one may not cross-pollinate them with one another (Shevet HaLevi 9:224). The primary halakhic position, however, is that the Torah forbade only the act of grafting two different trees onto one another, where they live together contrary to their natural order; it did not forbid cross-pollination, which occurs before the tree itself comes into being. Furthermore, a stamen on its own is not a species, since it cannot grow by its own power; likewise, the ovule in the ovary is not a species. Their hybridization is therefore not prohibited. All the more so, there is no prohibition of grafting in genetic engineering, which joins genetic material from one species with that of another (see also R. Yisraeli, Ha-Torah ve-ha-Aretz 3; Kerem Shlomo 2:1; R. Ariel, Teḥumin 33).
