LIFE HISTORY OF PlNUS 1 7 



CHAPTER I. 



MlCROSPOROGENESIS. 

 THE MICROSPORANGIUM^ 



The Wall of the Pollen-sac. With the exception of Pinus 

 Strobus, the staminate cones, in the pines which I have studied, 

 make their appearance in October or November. I have 

 searched repeatedly in the autumn for the male inflorescences 

 of Pinus Strobus but have never been able to find them until 

 late April or early May of the following spring. If they are 

 present at all before spring they can be scarcely more than 

 potentially so, for they are not sufficiently developed to be 

 detected in the field, nor by careful dissection in the laboratory. 

 /~The structure of the microsporangium agrees perfectly with 

 that usually described for the Abietinece. The wall of the 

 young pollen-sac consists of three or four layers of cells. The 

 cells of the outer layer are nearly isodiametric, while those of 

 the inner layers are smaller and more or less tabular in outline. 

 Just within, and in immediate contact with the archesporium, is 

 the ring of tapetal cells. In the early stages of development 

 the wall-cells are rich in cytoplasm and contain nuclei 

 which are large in proportion to the size of the cells. The 

 microsporangium increases much in size in the spring, and by 

 the time that the microspore-mother-cells are in the prophase of 

 division, considerable change has occurred in the wall-cells of 

 the pollen-sac. The outer layer has lost its nuclei and the cells 

 have become filled with a homogeneously staining resinous sub- 

 stance ; in Pinus Strobus this resinous deposit extends to the 

 second layer of wall-cells as well ; the cells of the inner layers 

 have been considerably flattened out, and their cytoplasmic con- 

 tent has become much reduced. When the pollen-grains are 

 mature, all the wall-cells of the microsporangium, except the 

 outermost layer, have disappeared. They have doubtless been 

 absorbed, their substance contributing to the nutrition of the 

 pollen-grains. 



The tapetum cannot be distinguished during the earlier stages 

 of development from the other tissues. It is first clearly differ- 



