288 



MORPHOLOGY OF GYMNOSPERMS 



as a food reservoir. Since there seems to be a general tendency 

 among gymnosperms to reduce the proembryonic stage of the embryo, 

 the Pinaceae may be regarded as a more advanced group in reference 

 to this character than the preceding ones. 



THE PROEMBRYO 



The development of the proembryo of Finns Laricio may be taken 

 as an illustration (figs. 341-349). The fusion nucleus retains its 

 central position in the egg, and organizes the largest spindle of the 

 free nuclear series. It is worthy of note that this first spindle is almost 

 always oblique (fig. 341). The two daughter nuclei are very small 

 ..^ at first, but increase rapidly in size, 



and then divide simultaneously. The 

 resulting four free nuclei, after increase 

 in size, pass to the base of the egg, 

 where they become arranged in a single 

 plane (figs. 343, 344). There follows 

 a simultaneous transverse division, 

 which results in two tiers of nuclei, 

 with four nuclei in a tier (figs. 344- 

 346). With this division the first 

 transverse walls appear in connection 

 with the spindles, and almost at the 

 same time vertical walls are formed 

 in each tier on secondary transverse 

 fiber connections estabhshed between 

 the nuclei (129) (figs. 350-352). This 

 appearance of walls at the eight- 

 nucleate stage of the proembryo is a 

 remarkably constant feature of the 

 Pinaceae. The older accounts of 

 Pinus introduce vertical walls at the four-nucleate stage, but it is now 

 discovered that a meshwork of fibrils was interpreted as a wall (fig. 

 350) . It must be understood that the first walls do not appear after 

 eight free nuclei are organized, but in connection with the organiza- 

 tion of the eight-nucleate stage. The lower tier of four cells is 

 completely walled, but the upper tier is open above, so that the nuclei 

 are freely exposed to the general cytoplasm of the egg. 





'-"^k<. 



'^WIS® 



Fig. 341. — Pinus Laricio: first 

 division of the nucleus of the fer- 

 tilized egg; the entire mitotic figure 

 is intranuclear; the two daughter 

 nuclei resulting from this division 

 are the first two nuclei of the sporo- 

 phytic generation; Xsoo. — ^After 

 Chamberlain (51). 



