You are growing plants hydroponically (rooted in an aqueous solution rather than soil). The solution for the control group of plants contains normal levels of salts and nutrients, while the solution for the treatment group has a high concentration of large molecular weight solutes that are excluded by roots. You find that the plants in the treatment group are on average smaller than those in the control group, but there is no significant difference between the groups in the amount of CO2 being fixed by the average plant. What differences at the cellular level between groups would you expect to make an important contribution to the difference in size of the shoots?