Yes they are shear webs and can be made of quite light materieal. On most planes a 1/16 inch sheet material is plenty. As to grain, it makes no difference whether the grain is vertical or horizontal. The actual sheer stresses are at a 45 degree angle to vertical and will switch directions 180 degrees as the strain goes one direction when the wings flex up and reverses to the other way when the wing flexes down. Check any strength and materials book on I beam design if you doubt this. The addition of the webs do greatly increase the rigidity of the wing with very little increase in weight.