[Chap. XXXVII HEREDITY IN PLANTS 443 



gametes by their union form the fertiHzed egg, which is the first cell of 

 each plant of the new generation. The gametes may be said to constitute 

 the bridge by which the hereditary factors pass from one generation to 

 the next one. 



Since the microspore is the forerunner of the pollen, and the mega- 

 spore is the forerunner of the embryo sac (Figs. 165 and 166), these 

 spores also constitute another single-celled bridge through which the 

 hereditary factors must pass during the life cycle of a seed plant. 



What is transmitted from parent to progeny? Hereditary potentialities 

 are merely the properties of certain units of matter. Hence certain units 

 of matter that pass from one generation to another in the sperm and egg 

 must be responsible for the hereditary resemblances and differences that 

 may be seen in plants. Any stable alterations of these units of matter 

 that change their properties would result in the development of heritable 

 variations, known as mutations. The differences in grouping of these units 

 of matter that occur in sexual reproduction account for the kinds of 

 hybrid variations exemplified by the flowers of snapdragon described 

 above. 



We may now consider the question whether there are anv visible 

 units of matter in cells that possess all the qualities necessary to account 

 for organic inheritance and also for heritable variations. From what has 

 already been said, it is evident that such units of matter must possess 

 certain definite qualities : namely, ( 1 ) they must be small enough that 

 several thousand of them may occur in a single cell, ( 2 ) their chemical 

 composition and organization must be sufficiently stable to account for 

 the fact that the distinctive characteristics of species and races of plants 

 have reappeared bv development in numerous succeeding generations 

 throughout centuries of time, ( 3 ) thev must reproduce during each cell 

 division in every growing region of the plant without losing their indi- 

 viduality, and ( 4 ) they must survive slight alterations in composition or 

 internal arrangement that account for occasional changes in their prop- 

 erties, or potentialities. 



Microscopic studies of cells for more than half a century have shown 

 that chromosomes may fulfill the last three of these conditions, but they 

 are too large and too few in number in the cells to fulfill the first condi- 

 tion. The cells of the common horsetail {Eqtiisetum arvense) have 272 

 chromosomes, the largest number known in plant cells. In some of the 

 fungi there are but 4 chromosomes per cell. The cells of some seed plants 

 have as few as 6. The majority of plants probably have less than 30 



