The substitution of a long vowel homogenous with the first, does not take place when a or ā is followed by a vowel (other than a) of the case-affixes of the Nominative and the Accusative.,
Thus वृक्षौ, प्लक्षौ, खट्वे, कुण्डे ॥ Here Rules (VI. I. 87) &c. apply. Why do we say \after अ or आ?\ Observe अग्नी ॥ Why do we say \followed by an इच् (a vowel other than अ)\? Observe वृक्षाः here (VI. I. I02) applies.,