Plants of Plantago lanceolata L. and Zea mays L., cv. ‘Campo’ were grown at two levels of light intensity. Especially in the roots, the rate of dry matter accumulation decreased at low light intensity. The carbohydrate content of both roots and shoots of P. lanceolata was not affected by light intensity. The relative contribution of SHAM1-sensitive respiration, the alternative chain, to total root respiration of both P. lanceolata and Z. mays, was not affected by light intensity during the daytime. The alternative pathway was somewhat decreased at the end of the dark period, but not in the root tips (0–5 mm) where it still contributed 56% in respiration. It was, therefore, concluded that photosynthesis is not a major factor in regulation of root growth in the species investigated. To see whether the effect of light intensity on root growth rate was via transpiration, plants of Z. mays were grown at different air humidities. Both high humidity and low light intensity affected the root morphology in such a way that the distance between the apex and the first laterals on the primary root axis increased. It is suggested that this effect on root morphology is due to transpiration and the subsequent removal of root-produced inhibitors of lateral root growth; although light intensity also affected the rate of dry matter accumulation of roots and the rate was not affected by the humidity of the air. It is, therefore, concluded that the effect of light intensity on the rate of dry matter accumulation of roots of Z. mays is not via an effect on transpiration.