Catherine Moore I think a ayer movement in plants one could be pretty nice, or a transpiration and xerophytes? Maybe haemoglobin? They would be my guesses…only guesses though!
Tissue fluid would be nice, but they usually like to have more than 6 marking points available… so: -High hydrostatic pressure at arteriole end -Caused by contraction of LV and narrowing of blood vessels -Ultrafiltration -More negative WP at veinous end -Water reabsorbed by osmosis -glucose delivered to tissues for respiration -urea and co2 removed -some tissue fluid absorbed by lymph I suppose that'd do it!