When I have fears that I may cease to be
Before my pen has glean'd my teeming brain,
Before high piled books, in charact'ry,
Hold like rich garners the full-ripen'd grain;
When I behold, upon the night's starr'd face,
Huge cloudy symbols of a high romance,
And think that I may never live to trace
The nature of Keats' fears are developed. 
 He continues the  Pastoral  imagery of the previous line.  The ‘full-ripened grain’ represents his poetry, and the Romantic idea of being at one with Nature is reinforced by the  alliterative , lush-sounding ‘g’s in 'glean’d’, ‘garners’ & ‘grain’. Keats’s mention of ripeness also suggests the fertility of his imagination. Note also the compound adjective ‘full-ripen’d’, a typically concise hyphened device that Keats uses frequently in his poetry. 
