What passing-bells for these who die as cattle?
Only the monstrous anger of the guns.
Only the stuttering rifles' rapid rattle
Can patter out their hasty orisons.
No mockeries now for them; no prayers nor bells,
Nor any voice of mourning save the choirs,--
The shrill, demented choirs of wailing shells;
And bugles calling for them from sad shires.
What candles may be held to speed them all?
Not in the hands of boys, but in their eyes
Shall shine the holy glimmers of good-byes.
The pallor of girls' brows shall be their pall;
Their flowers the tenderness of patient minds,
And each slow dusk a drawing-down of blinds.
Pallor  means paleness.  Pall  is a heavy cloth draped over a coffin; it has the power to produce an effect of gloom and grief. The two words are  consonant  and echo each other. Pall means a dark cloud of smoke, representing the Western Front, the result of gunfire, artillery barrages and gas attacks. 
 
 In effect the formal funeral, with mourning and flowers, was denied the soldiers on the battlefield. 
 The ‘patient minds’ is obscure and difficult to interpret, but it could refer to the long months and years of waiting for news of missing soldiers, ‘presumed dead’. Or else, it could refer to the forbearing population that accepted their bereavement stoically. 
 The last line is slow and almost monosyllabic, with long, stretched out vowels, and heavy ’d’s calling to mind death. The drawing-down of blinds used to be a custom in households where someone had died. Here, at dusk, it is of course a metaphor for lives that have ended.