326 THE PARTS OF A SEED-PLANT 



which will appear later, we are led to regard both pollen-sacs 

 and egg-sacs as cUstinct 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-pai't 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 fihoot. Thus in our flax embryo tlie 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 Lj'copodium (Fig. 1()6,;2). Pollen grains are spores; 

 and each egg-sac contains one or more comparatively large 

 spores within which an embrj^o 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 arc indicated in the accom- 

 panying diagram (Fig. 281). 



The tracing of homologies forms the basis of morphology, 



' Ilo-nmro-ffy <(li'. lumiox, same. 



