STUDIES OF IRRITABILITY OF PLANTS PEIRCE 79 



obvious. The available amounts of these decrease in the order named, the 

 supply of chlorophyll pigments being always very small but also very effect- 

 ive. The supply of carbon dioxide, because of its great dilution, rarely 

 reaches the optimum proportion, whereas the amounts of chlorophyll, light 

 and water, in proportion to carbon dioxide, often exceed it. Increasing the 

 supply of light beyond the usual amount without a corresponding increase 

 in the supply of carbon dioxide does not necessarily increase the products of 

 photosynthesis. But the foregoing experiments show that increased exposure 

 of chlorophyll-containing organs of liverworts, ferns and one grass to the 

 usual quantity of light results in increased growth. This increased growth 

 necessarily implies increased use of food, perhaps there may also be increased 

 food manufacture, but I do not yet know this to be the case. From this, 

 one is forced, so far as I can see, to conclude that light, up to a certain in- 

 tensity at least, stimulates growth rather than depresses it. 



2. We have seen, also, that a more uniform illumination increases sym- 

 metry in development. In certain instances this symmetry appears to be 

 complete, a dorsi-ventral giving place to radial structure, the stimulating 

 effect of light showing itself, not merely in change (increase) in size, but also 

 in change in form. 



3. The foregoing experiments on the ferns, Pteris aquilina and Gym- 

 nogramme triangularis, show that, although the archegonia and antheridia 

 ordinarily form on the shaded side of the prothallus, they form on both sides 

 when the illumination is equal, and in greater niunbers on the two sides than 

 is usual on either. Increased illumination is followed and accompanied by 

 increased numbers of the organs of sexual reproduction, the fertility of which 

 is evident from the development of sporophytes whenever there is sufficient 

 water for the sperms to swim. I am not aware that such experiments have 

 been made on ferns, but the experiments of Klebs^*, Vochting^^ and their 

 followers on algae, fungi and the flowering plants, demonstrate the intimate 

 connection between illumination and the development of reproductive organs, 

 light being for many plants the indispensable stimulus thereto. The behavior 

 of ferns is, therefore, consistent with the behavior of the other plants already 

 experimentally investigated. 



18 Klebs, G. Die Bedingungen der Fortpflanzung bei einigen Algen und Pilzen. 

 Jena 1896. 



19 Vochting, H. Ueber den Einfluss des Lichtes auf die Gestaltung und Anlage 

 der Bliiten. Jahrb. f. wiss. Bott. XXV. 1893. 



