86 
IOWA ACADEMY OP SCIENCE 
Are these tissues homologous throughout the various organs of the 
individual and in different individuals and species? 
Every cell of the root takes its origin from a single apical cell 
which is shaped like a three-sided pyramid, and cuts off segments 
on all four faces. Every cell of the stem originates from a similar 
apical initial which cuts off segments on three faces. The young 
leaf has for a time a similar tetrahedral initial with three planes 
of division, then the initial becomes “two-sided,” and after the 
rudiments of eight to eleven pinnae are established (in Dennstaed- 
tia punctilobida) the single initial is replaced by a group of apical 
initials of equal rank. Therefore, since each organ has its own 
type of initial, or may have several types in the course of its de- 
velopment, it is plain that the relation of a given tissue, the xylem 
for example, to the apical cell cannot form a final basis of homology. 
However, the apical cell or cells do not give rise at once to per- 
manent tissues. They form rather an embryonic tissue, the pri- 
mary meristem of Haberlandt, out of which the permanent tissues 
are gradually moulded. Now, in the leaf of Dennstaedtia puncti- 
lobida (Boulder Fern; Hay-scented Fern) the primary meristem 
may come from a two-sided initial, or from a group of marginal 
initials. Similarly, in the stems of true ferns (Filices), Maratti- 
aceae and Cycads primary meristems arise from a single tetra- 
hedral cell, or from a group of initials. But as no one questions 
the close phyletic relationship of these three groups, neither can we 
question the essential homology of their primary meristems. The 
origin of the primary meristems, therefore, need not figure in our 
discussion. 
Can we draw any inferences, then, from the manner in which 
mature tissues arise from this primary meristem? Hanstein long 
ago observed in certain flowering plants an early division of the 
meristem into three concentric zones: (1) the plerome or central 
portion which gives rise to the pith and vascular tissues; (2) the 
periblem surrounding this, which gives rise to the cortex of the 
mature stem; and (3) the dermatogen on the outside, which is the 
embryonic epidermis. The division between plerome and periblem 
falls just inside the endodermis. A similar alignment of cells in 
the primary meristem occurs in the roots of ferns (Filices). But 
the line of division is different in stems and leaves of ferns (at 
least in Dennstaedtia punctilobida) , where the endodermis is the 
outermost layer of plerome. Must we, then, believe that the endo- 
dermis of roots is not homologous with that of the stem and leaves 
of the self-same plant? I cannot so believe. But it is on such 
