248 GLOSSARY 



Megasporangium (large spore vessel). The sporangium which develops 

 megaspores. 



Megaspore (large spore). The larger one of the two sorts of spores pro- 

 duced by heterosporous pteridophytes ; it gives rise to a female 

 gametophyte. 



Megasporophyll (large spore leaf). A leaf bearing megaspores. 



Meristem (divisible). Formative, rapidly dividing tissue such as cam- 

 bium or the cells of growing points. 



Mesophyll (middle leaf). The entire parenchyma of the leaf, inside the 

 epidermis. 



Mesophjrte. A plant adapted to live with a moderate amount of soil 

 water and humidity. 



Micropyle (smaU gate). The small opening between the integuments 

 leading to the nucellus of an ovule. 



Microsporangium (small spore vessel). A sporangium which develops 

 microspores. 



Microspore (small spore). The smaller of the two sorts of spores pro- 

 duced by heterosporous pteridophytes ; it gives rise to a male 

 gametophyte. 



Microsporophyll (small spore leaf). A leaf bearing microspores. 



Mitosis (a thread or web). The process of indirect nuclear division 

 characterized by the presence of a spindle. 



Monocotyledonous. Having only one seed leaf or cotyledon. 



Monoecious (one household). Having the male and female sexual organs 

 borne separately by the same individual. 



Morphology (form discourse). The science of the form and structure of 

 an organism. 



Mutation. A decided and abrupt departure in the offspring from the 

 characters of the parent, often sufficient to constitute a new species. 



Mycelium (fungus growth). A mass of vegetative fungal filaments, or 

 hyphse. 



Nastic. A term applied to movements produced by all-round (not one- 

 sided) stimuli. The opening and closing of such flowers as the cro- 

 cus, tulip, etc., are thermonastic movements. 



Nectary. The organ in which nectar is secreted. 



Node (a knot). The part of a stem which normally bears a leaf or group 

 of leaves. 



Nucellus (a little kernel). The portion of the ovule within the integu- 

 ments and containing the embryo sac. 



Nucleolus (diminutive of nucleus). A small readily stained body, gen- 

 erally present with the chromatin, in the nucleus; also called a 

 nucleole. 



Nucleus (a kernel). The organ of the cell containing the chromatin and 

 nucleolus. 



Oogonium (egg offspring). The cell in the thallophytes which develops 

 the egg ; also called an oogone. 



