= Solution
For the positive-direction alternative, use the same paired statistic $t_{\rm obs}=2.607/(4.713/\sqrt{15})$, but reject at level $\delta$ only when $t_{\rm obs}>t_{14,1-\delta}$. Its one-sided $p$-value is
$$
\boxed{p_+=1-F_{t_{14}}(t_{\rm obs}).}
$$
Because the observed mean difference is positive and the null Student <probability distribution> is symmetric, the two-sided $p$-value is twice this upper-tail <probability>, consistent with the supplied $0.05$ and $0.025$.
The two-sided value is the null-model <probability> of a <test statistic> at least as far from zero in either direction as the observation. The one-sided value uses only the direction of larger cross-fertilized heights. They are probabilities under the assumed null <sampling distribution>, not posterior probabilities that the <null hypothesis> is true.
At the reported precision, the two-sided result is on the conventional 5 percent boundary; rounding can matter for a strict numerical threshold. The one-sided result is significant at5 percent and <supports> a positive mean difference if that directional alternative was chosen in advance. Choosing the direction after observing the sign would not justify using the smaller one-sided <probability> as a prespecified test. The paired design controls shared pot conditions, while the inference still relies on the <independence> and <probability distribution> assumptions for the differences.
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