The irregular participles 1. phulla 2. kṣība 3. kṛśa and 4. ullāgha are formed then only, when no preposition precedes them.,
The word फुल्ल is derived from the root ञिफला विशरणे the त is changed to ल ॥ The change of अ to उ 7.4.88 and the want of इट् augment 7.2.16 are regular. The same change takes place before क्तवतु also, as फुल्लवान् ॥
The affix त् is elided after the roots क्षीब्, कृश् and उत् -लाघ्, and the augment इट् is prohibited; this is the irregularity in क्षीबः, कृशः and उल्लाघः ॥
Why do we say when not preceded by a Preposition? Observe प्रफुल्लाः सुमनसः, प्रक्षीबितः, प्रकृशितः, प्रोल्लाघितः ॥ In the case of लाघ्, prepositions other than उत् are prohibited.
Or the augment इट् is added, and then इत् is elided from क्षीबितः &c. See 8.2.3 vartika
Vart:- The forms उत्फुल्लः and संफुल्लः should be enumerated. . Here त is changed to ल, though the root has taken a Preposition.
In the word परिकृशः, the word परि is not a Preposition with regard to the verb कृश्; परिगतः कृशः = परिकृशः ॥ So that परि is upasarga of the verb गतः understood, hence we have the form कृशः ॥,