1920] on The Earliest Known Land Flora 143 



The interest of the recent work on the modern Psilotaceaa centres 

 not so much in the details of the prothallus as in their embryology. 

 It has been shown by Holloway that the embryo of Tmesipteris is 

 rootless from the first. This suggests that the rootlessness is primi- 

 tive, and not the result of reduction. Since the Lower Devonian 

 plants were rootless also, it seems probable that this state was 

 characteristic of such early plants of the land. Further, the existence 

 of Sporogonites, and the very moss-like structure of its sporangium, 

 together with its similarity to the sporangia of Rhynia and Hornea, 

 seem to link up the latter naturally with the Bryophytes, which are 

 also rootless. In fact, we see before us a flora of rootless plants, 

 which raises afresh the question of the first establishment of the 

 neutral generation as an independent, soil-growing organism. It 

 originates in every case within the tissue of the sexual plant, and is 

 at first dependent upon it. This condition is seen in the embryo of 

 Tmesipteris, with details not unlike those of the Anthocerotea3. 

 How. then, did it first establish itself independently upon the soil ? 



This question was first raised long ago by Dr. Treub, the brilliant 

 Director of the Botanic Gardens at Buitenzorg. He suggested that 

 in the evolution of land-living plants a rootless phase would naturally 

 precede the full establishment of the sporophyte in the soil. He saw 

 this reflected in the embryonic state of certain Lycopods, where a 

 parenchymatous tuber precedes the establishment of the rooted plant. 

 It is attached to the soil by rhizoids, and contains a mycorhizic 

 fungus. This tuber Treub styled the "protocorm." He regarded 

 it as a general precursor of the established leafy plant in descent. 

 During the war new examples of this protocorm-stage have been 

 described by Holloway, which show the condition in its most pro- 

 nounced form. In Lycopodium laterale it constitutes the whole 

 plant-body for the first season. It bears numerous protophylls, and 

 may even branch, and reproduce itself vegetatively. It is only later 

 that the leafy shoot and lastly the root are formed. The fact that 

 Hornea shows a similar tuberous swelling at the base of the rootless 

 plant, and retains it even in the adult state, brings the added interest 

 that a permanent protocorm figures in the earliest known land flora. 

 Its antiquity is thus undoubted. But the Lower Devonian plants do 

 not all show it in a distended form. The tuberous swelling is not con- 

 spicuous in Rhynia or in Asteroxylon, and it is significant that in the 

 living Tmesipteris the rhizome is cylindrical. These facts indicate that 

 the distended protocorm is neither an obligatory nor a constant feature. 



It will not be necessary to do more than refer briefly to the 

 controversy whether the appendages of the Psilotaceae are truly 

 leaves or branches. The fact suffices that the question has been in 

 debate, and that similar questions arise in relation to the fossils 

 of the Lower Devonian. In them it is impossible to assign the 

 name leaf to any definite part in the full sense in which it is used 

 in the higher vascular plants. The difficulties of their morphological 



