342 BOTANICAL GAZETTE [October 



the difference in size means simply a difference in the nutritive 

 capacity, but further consideration shows this to be a hasty con- 

 clusion. • 



The important difference between the spores and gametes of 

 Ulothrix is that the latter pair and fuse and the former do not. To 

 obtain any suggestion as to the cause of this difference it is necessary 

 to recall the conditions of spore-formation and gamete-formation. 

 When the conditions favor maximum vegetative activity, neither 

 spores nor gametes are produced. When the conditions are less 

 favorable for vegetative activity, spores are produced;, and when 

 the plant is approaching the end of its activity, gametes are pro- 

 duced. It has been found possible to control experimentally the 

 conditions that determine these various activities. The distri- 

 bution of these functions in the ordinary life history of the plant 

 is naturally related to its changing environment. The production 

 of gametes in the simple plant we are considering is the last act 

 in the life of the plant, an act induced by conditions that are bring- 

 ing the activity of the plant to its close. 



But why do gametes pair and fuse ? It is obvious that more 

 important differences than a difference in size have been developed 

 in connection with the derivation of gametes from spores. The 

 difference in size is visible, but in connection with it there develops 

 a very different set of physiological conditions. This first stage 

 of gamete production has been called "isogamy," but the name is 

 only optically true. The same idea is expressed when such plants 

 are said to be "unisexual." Exception may be taken to both of 

 these terms. The gametes are alike in appearance, but that they 

 are not alike in fact is evidenced by their pairing and mutual 



attraction. Morphological likeness may justify the term isog 

 amy," but for the term unisexual there is no excuse whatever. 

 Sexuality, with its pairing sexual cells, implies two sexes, whether 

 they can be distinguished or not. All sexual plants must be 

 bisexual to be sexual at all. 



The result of the sex act in these primitive sexual plants deserves 

 attention. The zygote produced has the powers of an ordinary 

 spore, in that it can produce a new individual, but the notable 

 difference is that it does not germinate immediately. It is pro- 



