218 



THE ANATOMY OF WOODY PLANTS 



thickenings in their walls, resembling, in fact, short tracheids with 

 reticulate thickenings. By following the mass of reticulate mechan- 

 ical cells to the proximal end of the sporangium we find that they 

 are continuous with, and pass by imperceptible transitions into, 

 transfusion elements related to the fibrovascular bundles of the 

 sporophyll. In Ginkgo it is evident that the opening mechanism 

 of the sporangium is a derivative of the fibrovascular system, and 



does not take its origin from 

 the cells of the epidermis, as 

 is the case with the annulus of 

 the Cycadales, and forms lower 

 in the scale of vascular plants. 

 The situation is so important 

 in this respect that it is worthy 

 of being given a special nomen- 

 clature. In those types in 

 which the dehiscence depends 

 on epidermal mechanisms, in- 

 cluding the Pteridophyta and 

 the very lowest seed plants, 

 the term ectokinetic may be ap- 

 plied to the sporangium. On 

 the other hand, in the long 

 series of forms beginning with 

 Ginkgo and ending with the 

 monocotyledons, in which the opening mechanism is of internal 

 origin and related to the fibrovascular system, the designa- 

 tion endokinetic definitely indicates the origin of the apparatus 

 involved. 



It is necessary to examine more in detail the conditions found 

 in the walls of the sporangia of the forms above Ginkgo. In Fig. 163 

 is represented a longitudinal vertical section of the microsporan- 

 gium in Pseudolarix Kaempferi, a representative of the Abietineae 

 or pinelike conifers. Here the situation resembles in a general way 

 that found in the case of Ginkgo, for the opening of the sporan- 

 gial sac is due to the presence of reticulately thickened cells which 

 are likewise related to the fibrovascular system of the trace of the 



FIG. 162. Sporangium of Ginkgo biloba 



