Minnesota Plant Life. 523 



the egg outside the nucleus, while the head of the sperm fuses 

 directly with the nucleus of the egg. In cycads the swimming 

 lashes of the sperm are separated from its body immediately 

 after entering the egg and may be seen lying isolated in the 

 living substance around the nucleus. 



Behavior of the fecundated egg. After fecundation has 

 been completed the fused body is regarded as a single cell, so 

 perfectly have the tw^o components merged their identity in 

 their common product. This single cell is the fourth and last, 

 as here classified, of the important types of reproductive cells 

 formed by plants. i\fter a period of rest it normally segments 

 and grows, and by the continuation of this process a new 

 structure is brought into existence. This organism is known at 

 first under the general name of embryo, and matures into one or 

 another type of body, as may be determined in the species of 

 plant to which it belongs. Upon the whole, the behavior of 

 plant eggs is more various than that of animal eggs. Among 

 animals it is customary for the egg to mature into an organism 

 similar to one of those which, by the production of sperms or 

 unfecundated eggs, cooperated in the formation of the egg 

 itself. This is also true in certain groups of plants, as, for 

 example among the brown seaweeds, but is by no means the 

 general rule. In all higher plants, including the red seaweeds, 

 the higher fungi and the entire series from liverworts to seed- 

 plants, the egg does not grow into an individual capable of 

 producing sperms or eggs, but invariably into a spore-produc- 

 ing organism or its homologue. There are, therefore, in the 

 plant world two main types of egg segmentation, known re- 

 spectively as the direct and the indirect. When an egg seg- 

 ments directly it forms an embryo that gradually matures into 

 a sexual plant or its homologue. When an egg segments in- 

 directly it forms an embryo that matures into an asexual spore- 

 producing organism or its homologue. From the spores of 

 the asexual plant new sexual plants may arise, thus establishing 

 the phenomenon known as "alternation of generations." 



The extraordinary significance of alternation has already 

 been discussed in Chapter XIV, and need not again be elu- 

 cidated. In the simpler alternating life histories it would seem 

 that the first few cells of the young embryo become separated 



