forests, mountains, ravines and valleys

This phrase illustrates that the benefit of the Dharma extends everywhere, leaving no corner unreached.

all these plants being different in names and forms

Conditions (pratyaya) and physical forms are not identical.

were covered with a dark cloud

Cloud symbolizes the Dharma-body; rain represents the preaching of the Dharma. The Dharma-body pervades everywhere, fully and thickly, and the Dharma-sound permeates all over equally.

watered by a rainfall at the same time. {Everything rained on by the same cloud}

[The phrase] at the same time means that there is no [sequence of time], first or later. Raining down equally means that [rain] falls without any distinction between much and little. The Dharma rains of the four virtues with respect to li also are like that.

Rain “by the same cloud” implies that there is no water of a different taste. Although their genetic natures are varied, all [beings of different] appellations seek to sustain life. The same “rain” of the previous conversion [causes] the shoots of the Tao to issue forth: it is “the earth.” The present preaching helps them to attain understanding: it is the “rain.”