V. 



HOMOLOGY AXD AXALOGY" OP PLANT ORGANS. 



Br THE Ruv. George Hexslow, M.A., F.L.S., F.G.S. 

 A Lecture delivered at St. Albuns, IGth December, 1879. 



Homoh(j]i between any plant organs may be defined as an 

 identity of morphological origin with a ditference of function : 

 while Analogy signifies a similarity of function in different organs, 

 ■whether there be a difference in their morphological origin or not. 



Thus, a leaf-tendril is homologous with a leaf -blade, since they 

 are the same organ fundamentally and morphologically ; but each 

 in its development has become adapted to its own special function. 



The tendril of a vine, however, is not homologous with a leaf, 

 because it is a metamorphosed flowering branch, with which it is, 

 of course, homologous. On the other hand, it is analogous to the 

 tendril of the pea, for its function is the same. In this case, there- 

 fore, there is no common morphological origin between the tendril 

 of a pea and that of a vine. 



Again, recognising a green bract as an organ distinct, say, from 

 a petal ; though they are morphologically of the same nature, both 

 being referable to the leaf-type ; yet when a bract becomes coloured, 

 it is then analogous to a petal ; for its function is now the same, 

 viz to attract insects. On the other hand, whenever a bract is 

 green, it is, at one and the same time, both analogous and homologous 

 with a true leaf. 



It is the object of this paper to illustrate these principles as applied 

 to the Vegetable Kingdom. 



HOMOLOGY. 



A few preliminary facts must be stated. 



Every part of a plant to which some definite function can be 

 assigned is called an organ; and aU organs of a flowering plant 

 are grouped under the two heads, vegetative and reproductive. 



Under the former term are included roots, stems, stipules, leaves, 

 leaf-scales, and green bracts. Under the latter term are inflor- 

 escences, coloured bracts, flowers, and their resulting fruits. 



All these organs may be otherwise classified under the terms 

 axea and aiopendagcs ; the former being " caulomes " or stem- 

 structures, the latter " phyllomes " or leaf-structures and " tri- 

 chomes " or epidermal outgrowths. 



Roots and rootlets are called descending root-axes. These bear 

 no phyllomes, but only trichomes, as root-hairs for absorption. 



Stems may be (1) subterranean stem-axes or (2) aerial and 

 ascending stem- and branch-axes. 



Omitting hairs and other trichomes, stem- and branch-axes only 

 bear phyllomes or foliar appendages. 



VOL. I. — PART II. 4 



