There is luk elision of the case-affix of a word when it gets the name of dhātu 'root' or is of a crude form.,
A noun is changed into a root (धातु) when it is made a denominative verb. As पुत्रीयति 3.1.8 he wishes for a son of his own. A noun which forms part of a compound becomes a crude-form (1. 2. 46); as कष्टश्रितः 'having recourse to pain.' राजपुरुषः 'king's man.' In all these examples, the case affixes have been elided.
Thus पुत्रम् इच्छति = पुत्रीयति, here the 2nd case-affix is elided. कष्टश्रितः = कष्टश्रित &c.
Why do we say 'when it gets the name of : धातु or प्रातिपदिक ?
Because when it gets any other name, such as अङ्क &c., the case-affix is not elided. Thus वृक्षः, प्लक्षः ।,