GLOSSARY 351 
Inyolucre (rolled within). The leaf-like or bract-like sheath that in- 
closes a cluster of flowers. 
Irritability. The capacity which protoplasm possesses of respond- 
ing to stimuli, such as light, heat, gravity, and contact with chemical 
reagents. 
Isogamous (equal gametes). Applied to those plants whose pairing 
gametes are similar. 
Lamina (a layer). The blade or expanded part of a leaf. 
Leaf trace. The fibrovascular bundles from the leaf which blend with- 
in the stem with its fibrovascular cylinder. 
Lenticel. A round, oval, or lens-shaped opening on the exterior surface 
of the bark. 
Leucoplast (white molded). A minute colorless body within a cell. 
When exposed to light, leucoplasts may develop into chloroplasts. 
Liana. A climbing plant. 
Ligule (a small tongue). In grasses a thin appendage at the junction 
of leaf blade and sheath. 
Medullary. Relating to the pith; medullary rays are the pith rays 
which radiate to the bark between the fibrovascular bundles. 
Megasporangium (large spore vessel). The sporangium that produces 
the megaspores. 
Megaspore (great or large spore). The larger one of the two kinds of 
asexual spores produced by certain pteridophytes and all spermatophytes. 
Megasporophyll (large spore leaf). The leaf upon which the mega- 
sporangium develops. 
Meristem (dividing tissue). Tissues with the cells all nearly alike 
and still capable of subdividing. 
Mesophyll (middle leaf). The green or soft tissue of the inner part 
of the leaf. 
Mesophytes (middle plants). Normal land plants such as grow in an 
average soil and under a moderate climate. 
Metabolism. Chemical transformations of matter carried on by plants 
in the production and utilization of their food supply, and disposition 
of waste products. . 
Micropyle (small gate). The opening left by the integuments of the 
ovule, and which leads to the nucellus. 
Microsporangium (small spore vessel). The sporangium that produces 
the microspore. 
Microspore (small spore). The smaller spore of the two kinds pro- 
duced by certain pteridophytes and all spermatophytes. 
Microsporophyll (small spore leaf). The leaf upon which the micro- 
sporangium is borne. 
Midrib. The central or main rib of a leaf or thallus. 
