An investigation was made of the usefulness of old leaves and of some effects of light and nutrition upon their senescence. Three experiments were carried out on plants of Cucumis sativus grown in growth rooms and in a glasshouse. In the first, the effects of removing and shading old leaves suggested that they serve a useful function, not as photosynthetic organs, but as sources of substantial quantities of mobile elements which can be exploited to greater advantage by leaves in more favourably illuminated positions. In the second experiment, plants were grown horizontally so that individual leaves could be shaded independently of their neighbours. Shading hastened senescence. In the third, the effects of light intensity, defoliation, and nutrient deficiency upon the senescence of lower leaves were measured. The effects of light seemed to dominate those of nutrition in influencing the speed of senescence. The results are discussed in relation to the concept of the parasitism of lower leaves, the importance of redistribution of minerals, and the control of the course of senescence.