xi] REVERSION IN LATERAL SHOOTS 161 



plant in a greenhouse ; the leaves were of the mature form 

 differentiated into sheathing base, petiole and lamina except 

 in the case of a lateral shoot, which bore the grass-like, simple, 

 leaves which characterise the young plant. Goebel 1 also de- 

 scribes the occurrence of subdivided leaves of the water type 

 on lateral shoots of normal land plants of Limnophila hetero- 

 phylla. A corresponding reversion has been observed in the 

 case of the side branches of plants of Proserpinaca palustris* 

 developing in the air from a plant whose main stem was pro- 

 ducing the mature type of leaf; by removing the growing apex 

 of the stem in June, these side branches of the * water* type 

 were induced to develop. 



The interest of these lateral shoots, which show a reversion 

 to an ontogenetically earlier type of leaf, is enhanced by the fact 

 that C. and F. Darwin 3 have recorded a case of the occurrence, 

 on lateral shoots, of leaves whose characters are probably 

 phylogenetically earlier than those which the species normally 

 exhibits. Their observations related to the sleep habits of the 

 allied genera, Melilotus and Trifolium. They noticed, in Melilotus 

 Taurica, that leaves arising from young shoots, produced on 

 plants which had been cut down and kept in pots during the 

 winter in a greenhouse, slept like those of Trifolium^ with the 

 central leaflet simply bent upwards, while the leaves on the 

 fully-grown branches of the same plant afterwards slept accor- 

 ding to the normal Melilotus method, in which the terminal 

 leaflet rotates at night so as to present one lateral edge to the 

 zenith. They suggest that Melilotus may be descended from a 

 form which slept like Trifolium. 



The idea that the * juvenile' leaves, produced on lateral 

 shoots, may in some cases represent an ancestral type, is con- 

 sistent with the facts in the case, for instance, of the Alismaceae, 

 provided that the ' phyllode theory ' of the Monocotyledonous 

 leaf be accepted in the sense advocated by Henslow and the 

 present writer. According to this theory, which will be dealt 



1 Goebel, K. (1908). 2 Burns, G. P. (1904). 



3 Darwin, C. and F. (1880). 



A.W. P. II 



