182 GENERAL BIOLOGY 



two cotyledons the dicotyledons the leaves 

 are netted- veined, the stem has a hollow cylinder of 

 wood, and the flowers are usually in " fives " or 

 " fours." 



In plants, as in animals, the number of chromo- 

 somes is constant for any one species. When the 

 germ-tissue of the sporophyte develops the spores, 

 the cells which are to become mega spores and 

 microspores are found to have but half the number 

 of chromosomes that occurs in other cells of the 

 sporophyte. This reduction is accomplished, not 

 by the formation of chromatin " tetrads," as in 

 animals, but by a precocious splitting of the spireme 

 thread in the mitosis which precedes the formation 

 of the spore-mother cell. Four megaspores are 

 formed, which are usually arranged in a linear row. 

 Likewise, four microspores are formed, which are 

 arranged in a spherical mass and called by the 

 botanists " tetrads." These tetrads, composed each 

 of four adherent pollen grains, are to be carefully 

 distinguished from the chromatin figures in the 

 spermatocytes and oocytes of animals. 



Parthenogenesis in Plants. As in animals, so 

 in some seed plants, development may take place 

 without zygosis, a phenomenon that has been re- 

 ferred to previously as parthenogenesis. 1 Develop- 

 ment occurs by the spontaneous cleavage of the egg- 

 cell and the consequent formation of the embryonic 



1 The authentic cases are ThaUicirum (two species), Alchemilla 

 (nearly all species), Ficus hirta, and Antennaris alpina. Also one pine, 

 Pinus pinaster. 



