CONIFERALES 



28s 



It has been shown in so many cases that the tapetum, as far as it 

 is derived from either of these series, consists of modified wall cells, 

 that any claim of a sporogenous origin would have to be strongly 

 supported. Of course, much of the tapetum does not come from 

 either of these sources, but is differentiated from whatever cells hap- 

 pen to be in contact with the sporogenous cells. 



The development of the 

 sporangium in most conifers 

 is very slow, the young cone 

 becoming recognizable in the 

 spring, the archesporium ap- 

 pearing in early summer, and 

 the development of sporoge- 

 nous tissue continuing until 

 the autumn weather becomes 

 too cold, even for a conifer. 

 During the earlier stages of 

 development the cones are 

 entirely covered by scale 

 leaves (fig. 289). The actual 

 dates could not be given, 

 even for a single species, ex- 

 cept in some special locality; 

 for, the date for a special 

 stage, like the appearance of 

 the archesporium, for the appearance of the microspore mother cell, 

 for the reduction of chromosomes, and for the shedding of pollen, will 

 vary with the latitude, the elevation, and, in some cases, with the prox- 

 imity of warm or cold ocean currents. Many conifers are very success- 

 ful exotics. The times of shedding pollen of Cupressus macrocarpa at 

 Monterey, California, and at Auckland, New Zealand, differ by six 

 months. Since this naturally endemic conifer thrives from the lati- 

 tude of Monterey to that of Auckland, the pollen is doubtless shed 

 at various times, and other stages would vary correspondingly. How- 

 ever, in any particular locality, the same stages will appear at about 

 the same time, year after year. 



In the Chicago region, some conifers pass the winter in the micro- 



FiG. 288. — Taxus baccala: A, staminate 

 shoot with numerous strobili; B, single stami- 

 nate strobilus; C, ovulate shoot with two 

 ovules; A and C, natural size; B X2. 



