SYNGAMY 



235 



the centrally placed egg nucleus. ^^ In other cases^" the male cytoplasm 

 remains intact and surrounds the fusing sexual nuclei. The pollen-tube 

 cytoplasm often plays a conspicuous part in the formation of this 

 "mantle." 



The behavior of the chromosomes during the fusion of the gametic 

 nuclei and the first embryonal division has been described in a number of 

 conifers ; similar data for the cycads are as yet comparatively few. As a 

 general rule the chromatic elements of the two nuclei, although in the 

 reticular condition when the nuclei unite, do not become very intimately 

 associated in the fusion nucleus but remain distinguishable until the first 

 embryonal mitosis occurs. They then develop two groups of chromo- 

 somes which become arranged in a common achromatic figure-' (Fig. 143). 





I^Cv^ssT^JmS.' ^l^^ 



Fig. 143. — Syngamy in conifers. A, male nucleus pressing into female nucleus in 

 Pinus. B, first mitosis in zygote, showing parental chromosome groups. {After Ferguson, 

 1904.) C, parental chromosome groups in Larix. {After Wdycicki, 1899.) 



In Sequoia the male nuclei escape from their cytoplasm before their 

 discharge from the pollen tube, and after the nuclear fusion in the egg the 

 contributions of the two gametes cannot be distinguished (Lawson, 

 1904a). In gymnosperms, as in other organisms, all of the chromosomes, 

 paternal and maternal, divide longitudinally in the first embryonal 

 mitosis, the daughter chromosomes being distributed to the daughter 

 nuclei. 



Angiosperms. — The pollen tube, which is the elongated tube cell 

 bounded by the greatly extended intine, grows from the pollen grain on 

 the stigma through the style to the ovarian cavity and then by way of the 

 micropyle of the ovule through the nucellus and into the embryo sac 

 (Fig. 127). Usually the tube makes its way between the cells of the 

 style by means of enzymes which dissolve the middle lamellae of their 

 walls (Paton, 1921). After entering the sac the tube ruptures at its 



^^ Pinus (Ferguson, 1901, 1904), Thuja (Land, 1902), Juniperus (Norcn, 1904), 

 Cryptomeria (Lawson, 19046), Libocedrus (Lawson, 1907), Ephedra (Berridge and 

 Sanday, 1907). 



2" Taxodium (Coker, 1903), Torreya (A. Robertson, 1904; Coulter and Land, 

 1905), Cephalotaxus (Coker, 1907), Phyllocladus (Kildahl, 1908), Juniperus (Nichols, 

 1910), Agathis (Eames, 1913), Taxus (Dupler, 1917). 



^^ Pinus (Blackman, 1898; Chamberlain, 1899; Ferguson, 1901, 1904), Larix 

 (W6ycicki, 1899; Sm61ska, 1927), Tsuga (Murrill, 1900), Juniperus (Noren, 1907), 

 Cunninghamia (Miyake, 1910), Abies (Hutchinson, 1915), Bowenia (Lawson, 1926). 



