The below mentioned article provides an overview on the homology and analogy of organs with diagrams.
The study of the homology of an organ means the study of its morphological origin, development and position. Organs, however different in their outward appearance, are homologous if they are identical in morphological origin.
Analogy, on the other hand, refers to the outward appearance and the physiological behaviour of an organ. Organs are analogous if they are outwardly the same and perform the same function, however different their origins may be.
The tendril of pea is a modified leaf or leaflet while that of passion-flower is a modified axillary bud. So, these two tendrils are analogous (similar in outward appearance and behaviour) but not homologous (similar in origin). On the other hand, an axillary thorn and an axillary tendril are dissimilar in appearance but similar in origin and position. Hence, they are homologous but not analogous.
When we study the modifications of some plant organ, we study homologous structures. Thus roots and all forms of modified roots are homologous as all of them are roots by origin. Similarly, leaves and all their modifications are homologous.
On the other hand, when one studies particular types of organs, e.g., tendrils of different types, one finds that they are not all homologous. But, all tendrils are analogous organs.
Morphology may also be studied from another angle—by grouping together different analogous organs and then studying their homology.
Some such instances of grouping together are given below:
1. Homology of Climbing Organs which are all Analogous to Tendrils:
Under this head one groups together all types of tendrils and studies the homology of each of them.
(1) Tendrils of Vitis are modified apical buds , i.e., homologous to them.
(2) Tendrils of Passiflora are homologous to axillary buds.
(3) Tendrils of Lathyrus aphaca are homologous to whole leaves.
(4) Tendrils of Pisum sativum are homologous to leaflets.
(5) Tendrils of Clematis are modified leaf petioles.
(6) Tendrils of Gloriosa are homologous to leaf apices.
(7) Tendrils of Smilax are homologous to stipules.
(8) Tendrils of Antigonon and Cardiospermum are modified inflorescence axes.
(9) In the family Bignoniaceae tendrils are found which are homologous to rootlets.
(10) The homology of the tendrils of Cucurbits is not clear and has been supposed to be different by different authors. These tendrils are sometimes simple and sometimes branched.
Goebel’s opinion is that simple tendrils are modified prophylls of axillary shoots, while branched tendrils are modified extra-axillary buds in which the lower portions are modified lateral branches while the upper tendril branches are modified leaves.
2. Homology of Defensive Organs which are Analogous to Thorns and Spines:
Thorns and spines are different—the former being stronger structures. But all spines and thorns may be treated as analogous as they look similar and perform the same function.
(1) The thorns of Duranta, Bougainvillaea, Carissa , Flacourtia , Hygrophila, prune and the bigger thorns of Ulex are all modified axillary buds or stems.
(2) The thorns of Aegle marmelos and of Citrus spp. (lemons and oranges) are possibly modified prophylls.
(3) The spines of Acacias and Zizyphus are stipules.
(4) The spines of Opuntia , Asparagus , Berberis and the smaller thorns of Ulex are all modified leaves.
(5) The spines of Trapa bispinosa (water-chestnut) are modified persistent sepals.
(6) The spines of date-palm are apices of leaves or leaf segments.
(7) Spines of Punica granatum and Quisqualis indica are modified leaf petioles.
(8) Spines of Acanthorhiza and Pothos armatus are modified roots.
(9) Spines of Spinifex litloralis (Chapter LIV) are modified bracts.
3. Homology of Photosynthetic Organs:
Under these we may consider the phylloclades (including cladodes) and the phyllodes which are analogous to leaves performing the same function. The foliaceous stipules should also be considered here.
4. Homology of Storage Organs:
Different organs serve for the storage of reserve food material and so they are all analogous.
Under these one may consider:
(1) The underground stems like tuber, rhizome and corm.
(2) The fleshy scale leaves of underground bulbs.
(3) The fleshy leaves serving for storage of water arid food material.
(4) Fleshy cotyledons.
(5) Endosperm and perisperm.
(6) Bulbils of Dioscorea.
(7) Storage roots like root-tubers and other fleshy roots.
(8) Different parts of the fruit.