326 THE PARTS OF A SEED-PLANT 
which will appear later, we are led to regard both pollen-sacs 
and egg-sacs as distinct members of the plant body. We 
thus come to the conclusion that our typical plant viewed 
morphologically is made up of members of the nature of 
stem, leaf, root, pollen-sac, and egg-sac; and that the whole 
body may be furthermore regarded as -consisting of a chain 
of segments, each segment having at least a stem-part and 
a leaf-part and sometimes also other members. 
A root-member may be defined in a general way as typically 
a descending axis; a stem-member as an ascending leafy axis; 
and a leaf-member as a lateral, transversely flattened out- 
growth from a stem. Since stems and leaves imply one an- 
other, it is convenient to speak of them together as forming 
a shoot. Thus in our flax embryo the caulicle, cotyledons, 
and plumule constitute the shoot as distinguished from the 
root-part. A sac-member, such as a pollen-sac or an egg-sac, 
is really, as we shall see later, a spore-case essentially like 
that of Lycopodium (Fig. 166,2). Pollen grains are spores; 
and each egg-sac contains one or more comparatively large 
spores within which an embryo arises. Thus a sac-member 
is known by what it produces. As to how these different 
members may be further distinguished we shall learn more 
fully when we come to compare other plants with our type. 
100. Homologies. We have already seen that the terms 
analogy, analogue, and analogous, afford us a means of ex- 
pressing physiological equivalence or similarity in function. 
To express morphological correspondence or similarity in 
origin and position naturalists use the companion terms 
homology, homologue, homologous. Members of the same 
sort are said to be homologues of one another; any form of 
leaf-member, for instance, being homologous with any other 
form. Cotyledons and petals are homologues, because both 
are leaf-members, and they would accordingly be spoken of 
as homologous parts, homologous organs, or homologous 
members. The principal parts of our typical plant and their 
homologies as here understood are indicated in the accom- 
panying diagram (Fig. 281). 
The tracing of homologies forms the basis of morphology, 
1 Ho-mol’o-gy <Gr. homos, same. 
