122 



GYMNOSPERMS 



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fl:^ 



pressing the protoplasm against the wall of the cell. This single large 

 vacuole is filled with a colorless sap. Material from the outside passes 

 into the central cell, the amount of protoplasm increases rapidly, 

 and soon the entire cell is filled with a beautifully vacuolate proto- 

 plasm, with very large vacuoles in the central portion and smaller 

 and smaller ones toward the periphery. There is a gradual grada- 

 tion in the size of vacuoles from the large ones, more than 500 

 microns in diameter, down to the smallest ones, which can be seen 



with a 1.5 mm. oil immersion ob- 

 jective and a 20X eyepiece; and 

 probably the gradation continues 

 beyond the resolving power of 

 present-day microscopes. 



The central cell and its nucleus 

 grow for about 6 months before 

 the nucleus divides, the division 

 taking place about the middle of 

 April. The achromatic figure in 

 this division is rather scanty, and 

 there is not the slightest trace 

 of the formation of a cell wall 

 between the two nuclei (fig. 



123)- 



The ventral canal nucleus soon 



disorganizes, and, at the time of 

 fertilization, is usually unrecognizable. Occasionally, however, it en- 

 larges and goes through the same kind of development as the egg 

 nucleus. Sedgwick^" observed several cases of this sort in Enccpha- 

 larlos villosus, and suggested that the egg nucleus might be fertilized 

 by the ventral canal nucleus, as has been observed several times in 

 Ginkgo, Pinus, and Ficea. The fact that embryos have been found 

 in conservatory material where no male cones were present, indicates 

 that such a fertilization may occur. Hemsley's^'»' reported hybrids 

 between Ceratozamia longijolia and C. mcxicana, in which 3-year-old 

 pollen was used, also suggests such a fertilization; for cycad pollen 

 probably does not retain its vitality more than a month — probably 

 not so long. 



■rr.-fi.: 



Fig. 1 23. — Dioon edule: mitosis giv- 

 ing rise to the ventral canal nucleus 

 and egg nucleus, showing that there is 

 no cell plate between the two nuclei; 

 X3S0. — After Chamberlain.'"'' 



