HOMOLOGY OF TISSUES IN FERNS. 
BY HENRY S. CONARD. 
In comparing two or more organisms our standpoint may be 
developmental, morphological or physiological. Or, we may strike 
a middle ground in what is called the morpho-physiological point 
of view. 
Morphology and development agree in using the word homology 
to designate the relation between two organs which have a similar 
origin in the development of the individual, i. e., in ontogeny. For 
such organs we freely assume a common phylogenetic origin. An 
example of this is the relation of the spore mother cell of a moss to 
the pollen-spore mother cell of a flowering plant. They are ho- 
mologous organs. Purely physiological similarities are termed by 
common consent analogies. The relation between a tendril of a 
pea and that of the grape is only analogy. One is a modified leaflet, 
the other a modified shoot. The two organs have nothing in com- 
mon either in ontogeny or phylogeny, save the common irritability 
of their living substance. 
In all of the higher plants, however, there is continual new 
growth of organs from embryonic or meristem tissues. Thus a 
kind of serial homology is found, similar parts being successively 
formed anew in the growing regions. In such cases we find that the 
morpho-physiological basis of comparison is the truer one, and 
leads to sound phylogenetic conclusions. It is my intention to 
demonstrate this by reference to the structure and development 
of fern-plants (Pteridophytes) . 
First we shall see that similarity or difference in the origin of 
tissues from the apical cell or primitive meristem (Urmeristem of 
Haberlandt) is, in many instances, of no significance. It is well 
known that the roots, stems and leaves of ferns are traversed by 
continuous vascular tissues which serve to convey and distribute 
the sap, and that the details of these tissues are similar in what- 
ever organ they are found. The endodermis, phloem and xylem are 
perfectly distinct and homogeneous tissues. The epidermis also is, 
in most ferns, a continuous tissue all over the exterior of the plant. 
Only at the growing points are these tissue relations interrupted. 
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