There is only one plant in the center and not an entire vertical or horizontol row because, according to Rashi (see A-1 of our graphic), the only reason why "Rosh Tor" works to enable two perpendicular rows to be planted closer than 3 Tefachim to each other is when those two rows do not run into each other when either one of them is extended. For example, the row on the northern side, if extended towards the west, will not run into the row on the western side (since they both stop a bit before the corner). However, if one were to plant a row in the middle of the Arugah, even if he were to leave some space before reaching the side rows, "Rosh Tor" would not apply because if extended, this middle row would run directly into the side row.

Your question, however, did seem to have bothered the RASHASH (Daf 85a Rashi DH b'Memalei), who changed the wording of Rashi somewhat to conform to his understanding. His emendation, however, will not help to resolve your question, which is why we preferred not to accept his emendation or his understanding of the Rashi.

On the other hand, if we accept the interpretation of the Rashash, the answer to your question is that your point is discussed on 85b, where the Gemara asks if a furrow in middle of the Arugah "mixes up" or confuses the orderly Arugah (which it calls "Shurah"), causing its plants to become Kil'ayim. That is, for the order to be maintained in an Arugah the planting must be done in a very symmetrical manner. One furrow in the center of the Arugah ruins the symmetry (at least according to Rav Sheshes), and the plot becomes Kil'ayim. (According to Rav Ashi/Asi in that Sugya, indeed a single furrow can be plowed across the center of the Arugah, as you suggested. See also Rashash there.)

Be well,

-Mordecai