Hemiptera plant. Phloem sap, which has a higher concentration of sugars and nitrogen, is under positive pressure unlike the more dilute xylem sap. Most of the Sternorrhyncha and a number of Auchenorrhynchan groups feed on phloem. Phloem feeding is common in the Fulgoromorpha, most Cicadellidae and in the Heteroptera. The Typhlocybine Cicadellids specialize in feeding on non-vascular mesophyll tissue of leaves, which is more nutritious than the leaf epidermis. Most Heteroptera also feed on mesophyll tissue where they are more likely to encounter defensive secondary plant metabolites which often leads to the evolution of host specificity. Obligate xylem feeding is a
Y Sap mine effect of the British destructive artillery bombardment and recommended blowing Y Sap mine two days early, to destroy the "Blinddarm" and prevent enfilade fire from it during the British attack. The crater could be occupied by the British infantry and used by the Royal Engineers to dig an advanced jumping-off trench for the British infantry, which would reduce the distance the British infantry would have to cross by over . Hance's superiors agreed that firing Y Sap mine early could help to neutralise enemy fire from the flanks around La Boisselle but the explosion would alert the Germans that a