Studies in the Gesneriaceae of the Old World Xxxi. Some Aspects of Functional Evolution

A comparison is made between Old World Gesneriaceae, characterised by small seeds and an accrescent cotyledon, and other microspermous forest plants, which are usually mycorrhizal or wholly saprophytic.The early elaboration in the cotyledon of additional photosynthetic tissue, before the organization of plumule and leaf primordia, may be important in compensating for scanty food reserves.The unifoliate habit is seen as a normal kind of evolutionary development when viewed against the background of a group in which an accrescent cotyledon is already established.Monopliyllaea, confined to rainforest, is contrasted with Streptocarpus whose unifoliate species are able to survive in seasonal climates because of their ability to reduce leaf surface by formation of an abscission layer across the lamina.The evolution of the monocarpic habit in the family is briefly discussed.An examination is made of gesneriads with actinomorphic flowers.It is reasoned that their sporadic distribution in the family makes it unlikely that all are primitively actino morphic and evidence is presented for regarding some as being derived from zygomorphic ancestors.The terminology of floral symmetry is discussed.The Gesneriaceae of the Old World all have numerous, very small seeds : they also nearly all have some degree of persistent growth in one of the cotyledons after germination, leading to an eventual inequality in their size.For many years the term anisocotyly has been used to describe this condition.It is an accurate descriptive term that I have used myself in this series, but it lacks the appropriate biological emphasis for the present context.It is not the final inequality of size that is important, but the occurrence of con tinued growth.The first point in my thesis is that small seeds, numerous seeds and an accrescent cotyledon are interlocked.Salisbury (1942) has shown the importance of adequate seed reserves under forest conditions.Microspermy in the forest is often linked to a symbiotic association in the roots or complete saprophytism (Orchidaceae, Burman niaceae, Gentianaceae such as Cotylanthera, Monotropaceae etc.).The sym biont is needed at a very early stage in the life of the seedling and must help to compensate for the lack of food reserves in the seed.Is there any such compensatory factor in Gesneriaceae?A speculative suggestion is that the continued growth of one cotyledon may act in this way, by the production of additional photosynthetic tissue before energy is required to mobilize a plumular growing point and new leaves.