Taxonomic history:
   1753: Linnaeus described this species as Panicum alopecuroides in Species Plantarum, 1st ed., p. 55.
   today: The current name for this species is Pennisetum alopecuroides (L.) Spreng.

original Latin diagnosis
Panicum spica tereti, involucris setaceis fasciculatis unifloris, flosculo quadruplo longioribus.

English translation
Panicum with a spike round in cross-section, with involucres setaceous, growing in fascicles, one-flowered, [and] than a floret four-fold longer.

English translation with commentary

Panicum	Panicum
with a spike	spica - ablative singular of first declension feminine noun spica, -ae
round in cross-section;	tereti - ablative singular feminine of group B adjective teres, teres, tere; modifies spica
with involucres	involucris - ablative plural of second declension neuter noun involucrum, -i
setaceous,	setaceis - ablative plural neuter of group A adjective setaceus, -a, -um; modifies involucris
growing in fascicles,	fasciculatis - ablative plural neuter of group A adjective fasciculatus, -a, -um; modifies involucris
one-flowered,	unifloris - ablative plural neuter of group A adjective uniflorus, -a, -um; modifies involucris
[and]	[added for smoother reading]
than a floret	flosculo - ablative singular of second declension masculine noun flosculus, -i
four-fold	quadruplo - ablative singular masculine of group adjective quadruplus, -a, -um
longer.	longioribus - ablative plural neuter of comparative degree of group A adjective longus, -a, -um; agrees with involucris