Two Gsv- mutants have been isolated in mutagenized grain pea (Pisum sativum). A nuclear gene is affected in each mutant. The Gsv- plants are recessive homozygotes (gsv/gsv). They bear white stems and green leaves; the petiole, rachis, and veins on stipules and leaflets are also white. In the Gsv- plants, the stem is devoid of chlorophylls, although normal amounts of chlorophylla andb are present in leaflets. Mature chloroplasts and CO2 reduction ability are present in mesophyll tissue of leaves but absent in stem. The characteristics of Gsv- mutants allow the conclusion that the genes involved in chloroplast development and photosynthetic functions are regulated by different nuclear genes in mesophyll leaf cells on the one hand and green cells of stem and related tissues in functionally different segments of shoot on the other. It is proposed that segment-wise genetic control of photosynthesis in shoot is a rule among higher plants.