102 



SCIENCE. 



[N. S. Vol. XVII. No. 420. 



part of the plant. Such adventitious 

 shoots may arise from leaves, as in many 

 ferns, Begonia, Bryophyllum, etc. ; from 

 roots, in Ophioglossum, and many seed 

 plants, e. g., Populus, Bobinia, Anemone, 

 etc., or even from sporangia, as in the bud- 

 ding of the nucellus of the ovule recorded 

 in several eases of polyembryony. Now, 

 no morphologist would argue from these 

 that they are in any sense reversions, and 

 I can not see why the case of apogamous, 

 or aposporous budding is essentially dif- 

 ferent. 



No bryophytes have quite emancipated 

 themselves from the aquatic habit of their 

 algal progenitors. While they may often 

 dry up for an indefinite period without 

 being killed, there is, nevertheless, much 

 of the same dependence upon an ample 

 water supply that we find in the algffi. 

 Although much more resistant to loss of 

 water through transpiration than are the 

 few terrestrial algas, nevertheless the bryo- 

 phytes, as a rule, are much less suited to 

 a genuine terrestrial habit than are the 

 vascular plants. Much the same means 

 are employed by many bryophytes in the 

 absorption of water as by the algae. "Water 

 may be absorbed by all the superficial cells, 

 the roots playing a minor role as absorb- 

 ents, except in those forms in which the 

 plant is a prostrate thallus, where roots 

 are often developed in great numbers. 

 These delicate rhizoids, however, would be 

 quite inadequate to supply the needs of a 

 leafy stem of any but the most modest 

 proportions. In a few bryophytes, e. g., 

 Chimacium, there are rhizome-like modifi- 

 cations of the shoot, which may to a lim- 

 ited degree be compared to roots, but any 

 proper roots, like those of the vascular 

 plants, are quite absent. It would seem 

 as if nature's attempts to adapt the origin- 

 ally strictly aquatic gametophyte to a 

 radically different environment had been 

 only partially successful, owing to the fail- 



ure to develop an adequate root system to 

 restore the water lost through transpira- 

 tion. It may be that the range of varia- 

 tion any structural type may undergo is 

 limited.' 



If we accept this hypothesis, it may help 

 to explain the significance of the alterna- 

 tion of generations as developed among the 

 archegoniates, and we can understand why 

 the sporophyte has gradually replaced the 

 gametophyte as the predominant phase of 

 the plant's existence. Attention has al- 

 ready been directed to the perfectly well- 

 known fact that sudden marked variations 

 may appear in plants without any appar- 

 ent cause. The work of De Vries empha- 

 sizes this, and refers all radical advances 

 in structures to such mutations, which are 

 clearly distinguished from the variations 

 which occur within the limits of a species, 

 but which can not apparently overstep cer- 

 tain limits. 



In accordance with this view it is quite 

 conceivable that the first appearance of 

 the leaf upon the sporophyte may have 

 been comparatively sudden — that is, there 

 may not necessarily have been a long series 

 of preliminary structures leading up to a 

 true leaf. 



It has been urged that the antithetic 

 theory of the nature of the sporophyte 

 involves the sudden appearance of a new 

 structure. The fallacy of this claim has 

 been pointed out by Professor Bower, and 

 a little thought will show that no claim is 

 made of the sudden appearance of a new 

 structure. While no strictly intermediate 

 forms are known, there is certainly no dif- 

 ficulty in seeing the essential homology be- 

 tween the rudimentary sporophyte of such 

 an alga as ColeocJicete and that of Eiccia. 

 The antithetic theory merely claims that 

 the structure developed from the zygote, 

 which at first is devoted exclusively to 

 spore formation, gradually develops vege- 



