The importance of seed reserves for growth of Pinus resinosa Ait. during and shortly after seed germination was studied under controlled conditions. Tissues in the resting embryo were not completely differentiated. Many small, presumably reserve particles were present in the embryo in addition to reserves in the megagametophyte. During seed germination, procambia in the embryo first differentiated protophloem 2 days after seeds were sown. The radicle began to emerge from the seed coat at 5 days, at which time initial xylem formation was observed. Also, at approximately the same time, primordia of primary needles were forming in the peripheral zone of the apex. Elements of the photosynthetic apparatus, including stomata and mesophyll with chloroplasts, were differentiated first in the hypocotyl and then in cotyledons between 5 and 8 days after seeds were sown. Photosynthetic rates of young seedlings were correlated with rates of cotyledon expansion. During early developmental stages, reserve particles in megagametophyte cells and embryo cells gradually disappeared. Surgical removal of megagametophytes at various stages of seed germination resulted in subsequent growth inhibition of the hypocotyl-radicle axis, with early removal of cotyledons suppressing most growth. Growth of primary needles appeared to be influenced indirectly by megagametophyte reserves, probably by changes in amount of photosynthetic tissue. The embryo alone possessed capacity to differentiate such tissues as primary needle primordia, stomata, and primary and secondary vascular systems. Megagametophyte reserves appeared to contribute to growth of embryonic tissues only after the embryo itself initiated growth. Both current photosynthesis of seedlings and seed reserves contributed importantly to seedling development. © 1969 OXFORD UNIVERSITY PRESS.
