CYTOLOGY OF REPRODUCTION IN PLANTS 



155 



length to the egg membrane and appears to sink through it laterally 

 rather than endwise. This requu-es 20 to 30 minutes. The sperm 

 nucleus lies without conspicuous change in the egg cytoplasm for 24 to 

 36 hours, during which time the egg enlarges and becomes more highly 

 vacuolate. Then one end of the sperm nucleus penetrates the membrane 

 of the egg nucleus and at once begins to swell. This process continues 

 until the male nucleus is entirely within the egg nucleus and transformed 

 into a chromatic thread}^ mass ^\dth a nucleolus, these lying near the 

 corresponding elements of the egg nucleus. Eventually the sperm and 

 egg chromatin become indistinguishable. The egg continues to grow 

 and undergoes the first embryonal division about 6 to 9 days after 

 syngamy. In Pellia the male nucleus undergoes marked alterations as 

 it lies in the egg cytoplasm and shows a structure more or less like that 



Fiu. 113. 

 prophase 



— Syngamy in a liverwort {Kiccurdia) . 1, sperniatozoid. 2-4, syngamy. 5, 

 of first mitosis in fertilized egg. Description in text. (After A. M. Showaltcr.) 



of the egg nucleus by the time the two unite. Here, as in other groups 

 of organisms, there is some variation in the time relations of the various 

 events : penetration of the egg membrane by the sperm, structural altera- 

 tion of the sperm nucleus, nuclear union (karyogamy) , and mitotic 

 division of the zygote nucleus. 



The young sporophyte of liverworts contains in the capsule a largo 

 mass of sporogenous cells, some of which function as sporocytes while 

 others differentiate as hygroscopic elaters. The sporocyte divisions in 

 some species are noteworthy because of the four-lobed form assumed 

 by the cell before the meiotic nuclear divisions, which occur in rapid 

 succession at the center shortly before cytokinesis is carried to completion. 

 Other liverworts and some mosses show also a very regular behavior 

 of the plastids during these stages. In Anthoceros and Polyfrichum, for 

 example, the single plastid divides twice before the nucleus undergoes 

 its two divisions, with the result that each spore at first contains one 

 (Fig. 114). In Anthoceros every gametophytic and sporophytic cell 

 contains one plastid owing to this behavior at sporogenesis and to the 



