LIFE HISTORY OF PINUS II T 



part of May. The nuclei are thus separated, but no wall is 

 formed over the inner surface of the prothallium so each nucleus 

 is, as it were, enclosed in an open box. These cells stretch 

 out toward the center but never reach it without having first 

 divided by cell-walls from two to several times. The innermost 

 layer of cells always remains open on its free side until the cells 

 meet in the center and the endosperm becomes a continuous 

 cellular body. 



The spongy tissue becomes apparent as soon as the macro- 

 spore-mother-cell is differentiated, but it is not organized into a 

 definite zone with sharply defined limits until the beginning of 

 the second season of growth. 



These cells function as a physiological tissue of great impor- 

 tance in the nutrition of the 3^oung gametophyte. They doubt- 

 less convey nutrition derived from the disintegrating adjacent 

 nucellar tissue to the endosperm and are also occupied in the 

 manufacture of food materials. The spongy tissue doubtless 

 further serves to protect the young prothallium, not onl}'^ by 

 affording support but by driving out, as it were, the nucellar 

 tissue, thus making room for the delicate female gametophyte. 



The time at which the archegonia appear varies somewhat, 

 but in general they can first be detected about two weeks before 

 fertilization. They are normally found at the micropylar end 

 of the prothallium, and arise by the differentiation of certain 

 of the peripheral cells. By the later growth of the female 

 gametophyte, the mature egg is sunk to a considerable depth 

 in the prothallial tissue, but there always remains an open 

 channel leading from the neck-cells to the nucellar cap. The 

 number of archegonia varies in the different species from one 

 to nine. When the number of oospheres formed is small they 

 are almost spherical in outline ; but this shape may be greatly 

 modified according to the number and arrangement of the 

 archegonia. 



In Pimis Strobus the typical neck of the archegonium con- 

 sists of four cells, all lying in the same plane, while in Pimis 

 austriaca and P. 7'igida it is made up of eight, disposed in two 

 layers of four cells each ; but there is a lack of uniformity both 

 in the number and in the arrangement of these cells, not only 

 in different but in the same species. 



