354 GYMNOSPERMS 



An apical cell with definite segmentation is characteristic of the 

 higher Filicales, not only during embryogeny, but throughout the 

 whole life-history. In the lower Filicales apical cells are not so defi- 

 nite. In some of the lycopods an apical cell is very definite in the 

 early development, but later is replaced by a group of initials. This 

 behavior is also true of many fern gametophytes. So, it seems safe 

 to assume that the apical cell, as it appears in the embryogeny of 

 some conifers, is a primitive character, inherited from some fihcincan 

 progenitor, and appearing for a short time in the embryo. Those who 

 lay great stress upon recapitulation will include this apical cell in 

 their defense of the theory. 



At the stage of development shown in fig. 339 D, at least the lower 

 half of the embryo is meristematic. Later, the meristematic region 

 becomes localized, first near the base, where the root is being or- 

 ganized, and then higher up, where a meristematic region gives rise 

 to stem, cotyledons, and leaves. 



The meristematic group of cells of the young root is small. It 

 gives rise to the periblem and plerome of the root. The root cap 

 comes from the periblem. There is no dermatogen in the early em- 

 bryogeny (fig. 276). 



Investigators interested in vascular anatomy have begun their 

 study with the mature embryo, or later, and morphologists have 

 done their most intensive work on the proembryo. Some, like Buch- 

 HOLZ, have been adding to our knowledge of intermediate stages; but 

 these studies have not extended much beyond the apical cell and 

 early meristematic stages. 



From the appearance of the cotyledons to the mature seed, a con- 

 siderable amount of topographic work has been done. Polycotyle- 

 dony is prevalent in conifers, and there has long been a controversy 

 over its origin, some claiming that it is a primitive condition, while 

 others believe that it has been derived from dicotyledony by a split- 

 ting of the two cotyledons. 



Hill and De Fraine,''""'si relying principally upon vascular anat- 

 omy, claim that polycotyledony has been derived from dicotyledony 

 by a splitting of the two cotyledons. Sister Helen Angela made 

 a series of diagrams, based on vascular anatomy, showing intergrad- 



