462 



NATURE 



[December 9, 1920 



Evolution of Water Plants. 

 Waier Plants: A Study of Aquatic Angiosperms. 

 By Dr. Agnes Arber. Pp. xvi + 436. (Cam- 

 bridge : At the University Press, J920.) Price 

 315. 6J. net. 



WE have here a comprehensive work embody- 

 ing the results of a ten years' study of 

 water plants, and dealing in a very practical 

 fashion with the mass of literature that has grown 

 up around the subject. As a book of authority 

 on aquatic plants, it will be ranked with Schenck's 

 work of a generation ago, and it would not receive 

 its due if we did not add that it worthily holds 

 the place. But we must not forget that other 

 industrious investigators have in the meantime 

 filled the gap. Prominent among them are Gluck, 

 Goebel, Henslow, Pond, Sargant, Sauvageau, 

 Willis, and many others. Since Schenck's time, 

 however, new points of view have arisen, and new 

 methods have been in use by numerous inquirers 

 of both sexes, all keen in their desire to take a 

 part in the new era of research. 



Yet we find ourselves in an age of unrest and 

 uncertainty in the botanical world. There is a 

 note of discord in the rivalry of the two schools 

 of thought that are divided between the respective 

 claims of the present and the past, or, rather, of 

 the last and first stages of the evolution of the 

 higher plants, to occupy the attention of the in- 

 vestigator. Dr. Agnes Arber speaks of " the deep 

 obscurity involving all evolutionary thought" in 

 our own day, and demurs to the objection that 

 when we cannot even be sure as to the origins of 

 the numerous varieties springing up under our 

 eyes it is not a time for discussing the origins of 

 Dicotyledons or Monocotyledons. In this she is 

 impugning the policy of restricting our efforts to 

 the study of the last stage in the unfolding of 

 the story of plant life. So we get the keynote to 

 her book, the first six-sevenths of which are re- 

 garded by her as " a clearing of the ground for 

 the more theoretic considerations concerning the 

 evolutionary history of water plants." But in- 

 quiries into their origin must necessarily raise 

 great issues affecting also their relatives on the 

 land ; and thus we find in our hands a book that 

 in its general bearings will give birth to much 

 serious thought. 



Yet, aparl from theory, we have here a very 

 extensive record of observation, experiment, and 

 research on aquatic plants and their ways. The 

 practical side, as we have seen, occupies by far 

 the greater part of the work, and there is a wealth 

 of illustration, mostly original. Quite a third of 

 the space is devoted to the life-histories of the 

 NO. 2667, VOL. 106] 



various groups of aquatic flowering plants, and 

 these are followed by discussions on heterophylly, 

 the morphology and anatomy of stems and leaves, 

 the relation of the flowers to their environment, 

 the wintering of water plants, the physical factors 

 in their conditions of existence, and several other 

 matters. 



Taking our cue from the author, we will pass 

 on to notice the more speculative portion of her 

 book. There is first the question which might be 

 placed among the " posers " little children are 

 supposed to put. A primitive question about a 

 primitive issue is concerned with the priority of the 

 land- and the water-plant. As usually happens, 

 the problem needs a good deal of straightening 

 out before a reply can be made. Whilst it is 

 generally recognised that the primeval forms were 

 lowly aquatic plants, and that it is only in the 

 higher plants that the terrestrial habit has become 

 firmly established, we are far from being in a 

 position to connect the one with the other. 



Botanists are agreed that the aquatic Angio- 

 sperms are derived from terrestrial ancestors, but 

 a cleavage in opinion began when they differen- 

 tiated between Monocotyledons and Dicotyledons. 

 Nature seemed to differentiate between them, and, 

 being impressed by the preponderance of aquatic 

 families among the Monocotyledons, botanists 

 accredited the class with special aquatic proclivi- 

 ties. After considering the matter more closely, 

 the author forms the opinion that these tendencies 

 are hard to maintain. 



Continuing to treat aquatic .Angiosperms in the 

 mass, and heeding only the systematic distribu- 

 tion of aquatics, the author lays down the prin- 

 ciple that when a single genus or a species in an 

 otherwise terrestrial family has taken to aquatic 

 life, the habit may be a recent one ; but when a 

 whole family holding several genera is aquatic, we 

 are dealing with a very ancient group of water 

 plants where "the differentiation of the genera has 

 occurred since the adoption of the aquatic habit." 

 There are, in fact, "aquatics, new and old"; and 

 they tend to choose their places in the systematic 

 scale according to their age. the more ancient 

 among the more primitive of the Angiosperms, 

 and the more recent among those more ad- 

 vanced. 



To the query as to which are the most primitive 

 of the Angiosperms, the answer is : The families 

 that are held within the Ranalean plexus. Those 

 numerous families that go largely to form the 

 Incompletae of Bentham and Hooker, and which 

 Engler places at the beginning of the Archi- 

 chlamydese in the scale of development, are re- 

 garded as the more advanced and reduced forms 



