GLOSSARY 437 



Irregular flower. One showing lack of uniformity among members of the same whorl 



of perianth parts. 

 Isocarpic. Flowers in which the number of carpels is equal to that of each of the other 



cj'cles. 

 Isogamete. A gamete without apparent sexual differentiation. 

 Isogamous. Characterized by the union of gametes of equal size. 

 Isomorphic. Of like form ; a type of alternation of generations in which the gameto- 



phyte and sporophyte are similar vegetatively. 

 Isthmus. A contracted part or passage connecting two similar structures or cavities, 



as in desmids. 



Lamarin. A dextrin-like carbohydrate constituting a form of reserve food in many 

 brown algae. 



Leaf gap. A break or interruption in the continuity of a siphonostele caused by the 

 departure of a leaf trace from it. 



Leaf trace. A strand of vascular tissue passing through the cortex of a stem and con- 

 necting the stele with a leaf base. 



Leaflet. One of the parts of a divided ("compound") leaf 



Lenticel. A pore in the corky bark of a woody stem through which gases pass. 



Leptosporangiate. A type of sporangial development, occurring in the higher ferns, 

 in which the sporogenous tissue arises from the outer segment of the initial cell. 



Leucosin. A protein-like substance constituting a form of reserve food in certain 

 algae. 



Lignin. A substance related to cellulose and with it forming the cell walls of woody 

 tissue, stone cells, and most fibers. 



Ligule. A single scale-like appendage near the base of some leaves on the adaxial sur- 

 face, as in Selaginella and Isoefes. 



Linear. Long and narrow with parallel sides. 



Locule. A compartment or cavity, as in a sporangium or ovary. 



Megaphyllous. Having large leaves. 



Megasporangium. A sporangium producing only megaspores. 



Megaspore. In heterosporous plants, a spore that gives rise to a female gametophyte 

 and generally is larger than a microspore. 



Megasporophyll. A sporophyll that bears only megasporangia. 



Meiosis. The two nuclear divisions that result in a reduction in chromosome num- 

 ber from the diploid to the haploid condition. 



Meiospore. A spore formed as a result of meiosis. 



Meristem. Embryonic tissue, the cells of which are capable of active division. 



Mesarch. Development of primary xylem in both centripetal and centrifugal 

 directions. 



Mesophyll. Green parenchyma occurring between the epidermal layers of a leaf and 

 forming the internal ground tissue. 



Metaxylem. Primary xylem formed later than the protoxylem. 



Microphyllous. Having small leaves. 



Micropyle. A minute opening in the integument of an ovule through which the pollen 

 tube ordinarily enters. 



Microsporangium. A sporangium producing only microspores. 



Microspore. In heterosporous plants, a spore that gives rise to a male gametophyte 

 and is generally smaller than a megaspore. 



Microsporophyll. A sporophyll that bears only microsporangia. 



