EVOLUTION OF PLANTS.

Otago Daily Times, Issue 19791, 17 May 1926, Page 12

EVOLUTION OF PLANTS.

W.E.A. BIOLOGY CLASS. The fourth of a series of lectures of th W.E.A. class in biology was held on Friday night in the Upper Oliver Room, University Buildings, the tutor being the Rev. Dr J. E. Holloway, F.N.Z.1., lecturer in botany of the University of Otago. Judging from the increased attendance since this class commenced, it shows ample evidence of the appreciation by the citizens in the educational opportunities offered them by the W.E.A. Association and all those who labour so worthily for the cause. Following on from the last lecture, when it was shown that the algrn are adapted altogether to life in the water, both as regards the vegetative structure of the plant body and also with respect to their methods of reproduction —namely, asexual spores and sexual gametes. Dr Holloway proceeded to consider the liverworts and mosses, which are the lowliest of existing land plants. In their life history, he said, there is a regular and definite alternation of a sexual gamete-bear-ing stage with an asexual spore-bear-ing stage. With the aid of lantern slides Dr Holloway showed in a very clear and distinctive manner that in some respects these plants have become adapted to life on land, whilst in other respects they show an adaptation to the. primitive life in the water. Taking the marchautia, which is one of the common types oi liverworts growing in damp places, it will be seen on examination that it has a flat ribbon-like plant body, comparatively simple in structure, but with air chambers and breathing pores. At special points on the margin the ribbon grows up erect and forms umbrella-shaped structures on which are borne the gametes in special made and female .organs. The male gametes are swimming sperms which need the presence of water in order to reach the eggs. The fertilised eggs prow into the asexual spore-bearing stage. This takes the form of a many-celled, stalked capsule, which sticks out into f the air from the female umbrella, and distributes its dry non-swimraing spores by the wind. Each spore can germinate on - damp soil into a new gamete bearing plant. Thus the gamete-bearing stage is dependent upon the presence of water for fertilisation, and the spore-bearing stage must be held up into the air for the distribution of the spores. In certain other liverworts we see some very significant examples of the progressive specialisation of the spore-bearing stage to the dry air conditions of life on the land. Passing on to the mosses, it is shown that they flourish best in damp places, but many of them have adapted themselves to drier conditions than have liverworts. Polytrichum is a very common moss. The gamete-bearing stage consists first of a little mat of branched filaments, not unlike a simple alga, and from this there arises one or more erect leafy stems. At the apex of these stems are borne the male and _ female organs with their gametes. The male gametes are swimming sperms as in the liverworts. The fertilised egg at the apex of a female branch grows out into the spore-bearing stage. This is much more specialised to sub-aerial conditions in mosses than in liverworts. In polytrichum it consists of a capsule held high up into the air on a ■ long stalk, and it possesses both breathing pores and aeration tissues, and also an elaborate mechanism for scattering the spores into the air. When the spores germinate they give rise to the .gamete-bear-ing stage—that is, creeping filaments with et;ect leafy stems. Thus in mosses both the gamete-bearing and the sporebearing stage is more adapted to a subaerial life than the liverworts, but the former gamete is still dependent upon the presence of water for its creeping filaments, and especially for its swimming sperms. Other mosses, such as spagnum, the bog-moss, show essentially the same amphibious alternation of asexual generations, or, as often more commonly expressed. one foot in the water and the other foot on the dry land. In summing up. Dr Holloway showed that the outstanding feature in the life history of liverworts and mosses, compared with that of the alga;, is the definite alteration between the, sexual and asexual method of reproduction. The latter has become adapted in the liverworts to the sub-aerial life, and still more so in the mosses, although the method of sexual reproduction in both still retains, in part at least, the acquatic habits of the algal ancestors of these land plants. The significance of this alteration with respect to the plants concerned is clear, but there are differing theories as to how it actually arose.