Megaspore wall growth in Selaginella (Lycopodiatae)

Abstract

Different stages of megaspore and megasporangial development inSelaginella argentea (Wallich)Spring,S. bigelowiiUnerw., andS. kraussiana (Kze.)A. Br. have been seen and studied. Megaspore wall units give positive reactions for polysaccharides and protein in young megaspores, and become the thick and resistant wall typical of the genus only later.—Units forming the exospore and the spaces between units enlarge from widths of 5–10nm early during development up to over 200 nm at pregermination stages. The spaces enlarge first. Initially they are circular and mostly about 70 nm in diameter. Later, spaces toward the inner part of the exospore enlarge more than those near the outer surface. During pregermination, wall spaces range in size from 4 to 50 times the width of units with the larger spaces located near the inner surface. As a result the exospore would be under tension to spring outward during germination when the laesurae are lysed.—A gap in the exospore, shaped like a half-moon in polar sections, forms in equatorial and distal portions of the spore. This gap becomes enormous, three times the volume of the central space plus the mesospore, and is filled with lipids and other nutrients. Late in development, during the period of tapetal cell degeneration, the gap contents are moved into the central space and the gap is closed.—Late in development the mesospore is degraded. Its products, along with gap contents, seem to be added to the contents of the central cavity and appear as reserve storage globules. A primary wall-like endospore is formed during this period, at the inner surface of the exospore. During germination this endospore develops further at its inner surface.—Changes in the size and shape of megasporangia occur independently of the size of megaspores.