The u is the substitute of ru (the r substitute of a final s [[8.2.66]]) when an a , which is not a pluta, both precedes and follows it.,
The phrase उत् is understood here, as well as the word आत् of 6.1.109, 6.1.111. Thus वृक्ष + सु (Ist singular) = वृक्षस् = वृक्षर् 8.2.66. वृक्षर् + अत्र = वृक्षउ + अत्र = वृक्षो + अत्र 6.1.87 = वृक्षोऽत्र 6.1.109; so also प्लक्षोऽत्र ॥ This ordains उ for र्, whereby 8.3.17, there would have been otherwise य; and this उ does not become asiddha (as it depends upon रु 8.2.66) for the purposes of 8.3.17, as it otherwise would have been by 8.2.1.
Why do we say 'after an अ'? Observe अग्निर् + अत्र = अग्निरत्र ॥ Why do say 'a short अ'? Observe वृक्षाः + अत्र = वृक्षा अत्र ॥ Why do we use रु with its indicatory उ, and not use the र् generally? Observe स्वर् + अत्र = स्वरत्र, प्रातर् + अत्र = प्रातरत्र ॥ Here the final र् is part of the words, and is not produced from स् ॥ The word अति is understood here also from 6.1.109. The र must be followed by a short अ, therefore, not here: वृक्षर् + इह = वृक्ष इह ॥ The subsequent अ must be short, the rule does not apply here वृक्षर् + आश्रितः = वृक्ष आश्रितः ॥ Why do we say 'preceded by an apluta अ' ? The rule will not apply if a Pluta vowel precedes it. As, सुस्रोता ३ अत्र त्वमसि 8.2.84. Why do we say \when followed by an apluta अ\? Observe तिष्ठतु पय आ३ग्निदत्त 8.2.86. Here प्लुत being held asiddha, there would have been उ substitution, had not the phrase अप्लुते been used in the aphorism.,