276 PLANT PHYSIOLOGY 



to change their behavior accordingly. So, also, changes 

 of other sorts, making the conditions suitable not only for 

 the growth and healthy life of the individual but also for 

 the production and active life of new individuals, will induce 

 plants to form these by sexual or by non-sexual means, 

 according to the species of plant. In the spring, when the 

 conditions for active vegetation are becoming more and 

 more favorable, the Equisetums complete the development 

 of the spores non-sexually formed in the foregoing season, 

 and shed these spores, which germinate at once, if at all, 

 and form new plants, the prothalli. These, if conditions 

 favor, soon develop sexual organs and produce new plants 

 by sexual means. In these alternating generations, the non- 

 sexual and the sexual, which follow one another in suc- 

 cessive years, the conditions of the later growing period 

 of one year induce the mature plants to form spores by 

 non-sexual means. The different conditions of the earliest 

 growing period of the following year induce the same plants 

 to complete the development of the spores already formed. 

 The still different conditions of the days and weeks suc- 

 ceeding induce these spores to germinate, the prothalli to 

 form archegonia and antheridia, the embryos to develop 

 into the larger non-sexual plants. These last vegetate 

 for an indefinite length of time. The successive changes in 

 the seasons, in the soil, and in the water-supply, fix the 

 succession of generations which recur according to the 

 peculiar adjustment of the species to all the factors of its 

 environment. Species, genera, families, and orders differ in 

 their adjustments and in their reactions to the stimuli given 

 by the many different factors composing the environment. 

 Comparing sexual and non-sexual reproduction, we see 

 that the advantage of the parents determines what mode 

 of reproduction, if any, shall be carried out. For a long time 

 it has been supposed that sexual reproduction is distinctly 

 advantageous to the species, if not occasionally altogether 

 necessary to those forms capable of reproducing themselves 

 by this means. Such a view is not quite correct. Many 

 species of plants, among them forms which are eminently 

 successful as judged by their numbers and activity, are 



