THE ORIGIN OF SEX IN PLANTS. 73 



produce zoospores if grown in solution of inorganic salts and in bright 

 sunlight. Nets with strong inclinations to form zoospores can be 

 made to produce gametes by cultivating in a sugar solution in subdued 

 light or darkness. Plants that have no special inclination to form 

 either zoospores or gametes may be decided one way or the other by 

 the illumination, bright light producing zoospores and darkness 

 gametes. It is also fair to say that sometimes the tendency to form 

 zoospores is so strong that a plant will not yield for several generations 

 to the conditions that generally bring about the immediate production 

 of gametes. 



Let these studies on Hydrodictyon and Ulothrix stand as illustra- 

 tions of the kind of evidence presented in varying degrees by many 

 algae and fungi and constantly increasing as investigations in physiol- 

 ogy proceed. The general trend seems unmistakable. We may feel 

 sure that sexual elements, gametes, have arisen from asexual repro- 

 ductive cells with an immediate relation to and probably because 

 of certain environmental factors. In a general way these factors 

 are known to be light, temperature, osmotic pressure and, most im- 

 portant of all, the chemical nature of the environment with especial 

 reference to the kinds of foods. 



What was the change that came over the asexual reproductive cell 

 when it took on the stamp of sex? The differences are best measured 

 in the possibilities of the two elements. The asexual zoospores may 

 quickly and readily produce a new individual. The gamete, generally 

 speaking, must fuse with its kind or else die. We have seen that primi- 

 tive gametes may germinate without conjugation but the resulting 

 plants in the cases best known are weaker than normal individuals. 

 We also know that the lower stretches of the plant kingdom furnish 

 abundant illustrations of parthenogenesis, that is, the power of an egg 

 cell to develop without fertilization. These exceptions, however, 

 strengthen the evidence that the essential differences between gametes 

 and asexual zoospores are qualities lacking in the former, and especially 

 the ability to continue and sustain the mechanism demanded by vital 

 processes. 



With conjugation all is changed, and the sexually formed spore 

 has the qualities lacking in the two gametes from which it arose. 

 The protoplasm is in a sense rejuvenated and with the stimulus comes 

 sooner or later an expression frequently more vigorous than that of 

 the asexual spore. 



The most striking conjecture on the significance and origin of sex 

 has been presented under the name 'autophagy.' It is a very simple 

 hypothesis. However, its simplicity is its greatest danger and will 

 probably be its complete undoing, for enough is known to indicate 

 that the factors and conditions that produce the sexual act are im- 



