Before a case-ending beginning with bh or s (literally a consonant of the jhal pratyahāra) in the plural, e is substituted for the final a of Nominal-stem.,
As वृक्षेभ्यः, प्लक्षेभ्यः, वृक्षेषु, प्लक्षेषु ॥ Why in the Plural? Observe वृक्षाभ्याम् प्लक्षाभ्याम् ॥ Why before a case-affix beginning with a झल् consonant? Observe वृक्षाणाम् (the lengthening here is by 6.4.3). Why a case-affix? Observe यजध्वम्, पचध्वम् ॥,