THE MICROSPORANGIUM 



219 



microsporophyll. As in Ginkgo, the transition from the mechan- 

 ical tissue to the tracheids of the fibrovascular bundle is effected 

 by transfusion elements. An interesting difference between the 

 situation found in Ginkgo and that illustrated in the Abietineae and 

 other conifers is the fact that the mechanical elements invade the 

 epidermis in Pseudolarix, but fail to do so in the more primitive 

 genus. In other members of the Abietineae, particularly where 

 the wall of the sporangium is very thin, the mechanical tissue 

 becomes correspond- 

 ingly reduced in 

 amount and no 

 longer shows any 

 clear relation to the 

 fibrovascular sys- 

 tem. In the sub- 

 tribes of Coniferales 

 above the Abie- 

 tineae this condition, 

 in fact, becomes the 

 rule, and so abortive 

 does the mechanical 

 layer become that it is represented by the reticulately thickened 

 cells of the epidermis alone. It thus results that the opening of the 

 sporangial cavity is once more effected by superficial cells, but the 

 situation here represented should be carefully distinguished from 

 that in the Pteridophyta and Cycadales, since it is the result of the 

 invasion of the epidermis by mechanical tissues of fibrovascular 

 origin. Subsequently, when the dehiscing mechanism was reduced, 

 the epidermis once more became secondarily the seat of the opening 

 device. It is not without significance in this connection that the 

 araucarian conifers, which are often regarded as the lowest, present 

 the sporangial arrangements of the mass of conifers in which the 

 dehiscing mechanism is reduced to its lowest terms, and not that 

 luxuriant and apparently primitive condition exemplified by Ginkgo 

 and the Abietineae. 



In the Gnetales the small sporangia are not characterized by the 

 presence of a very well-developed opening device. The situation 



FIG. 163. Sporangium of Pseudolarix Kaempfcri 



