242 MORPHOLOGY OF GYMNOSPERMS 



the mature sporangium wall, after the breaking -down of the inner 

 layers, consists of two layers (epidermis and endothecium), in Wid- 

 dringtonia (159) the mature wall of the small sporangium becomes 

 one-layered, all the true wall layers within the epidermis disappearing. 

 The primary sporogenous layer, by successive divisions, produces a 

 mass of sporogenous cells, which finally reach the mother cell stage, 

 at which time the tapetum forms a complete investment of glandular 

 and sometimes binucleate cells. 



In most of the recorded cases, the microsporangium passes the 

 winter approximately in the mother cell stage (figs. 266-269). This 

 is certainly true for Cupressus Lawsoniana (48), Pinus silvestris 

 {126), Larix europaea (74), Taxodium (76), and Juniperus communis 

 (173). In her study of Pinus, however. Miss Ferguson (87) states 

 that in most species, while the sporogenous tissue is well developed 

 before winter, the mother cell stage is not reached until the following 

 spring; while in P. Strobus even the sporogenous tissue is not well 

 developed until May. In Cryptomeria japonica (in California) 

 Lawson (93) found that the staminate cones appear early in Octo- 

 ber, the reduction divisions occur during the first week of November, 

 and the microspores are rounded o£E by December. In Juniperus 

 virginiana in the vicinity of Chicago, the pollen mother cell divides 

 before development is checked by the cold weather. Observations 

 have not been recorded for tropical species, but we have seen the 

 Mexican Pinus patula ( ?) shedding pollen in September, which would 

 indicate that the pollen had been matured without any resting period. 



These discrepancies show a wide seasonal variation in the develop- 

 ment of the sporogenous tissue. Even in the cases first cited there 

 is evident a certain amount of variation in seasonal range and rate of 

 development. For example, in Pinus Laricio and Larix europaea the 

 mother cell stage is reached usually in October; while in Taxodium the 

 staminate cones do not begin to develop until September or October, 

 but they are in the mother cell stage at the arrival of winter (January or 

 earlier in the southeastern United States) ; in both cases the mother 

 cell stage is the winter stage, and the reduction divisions take place 

 during the following spring (March to May), but in an early winter 

 Pinus Laricio may not reach the mother cell stage, and a few sporo- 

 genous mitoses will then precede the mother cell stage the next spring. 



