FERTILIZATION IX GYMNOSPEEMS. 



439 



finds there a certain amount of moisture by means of which a 

 tube is formed from one of the large cells. This extends directly 

 into the tissue of the nucellus, coming sooner or later into con- 

 tact with the summit of the embryonal sac, and then affecting 

 the corpuscules below. From the fertilized corpuscule the embryo 

 is developed. 1 



1 For the purpose of affording some means of comparison of the methods of 

 reproduction in flowering plants and in those of a lower grade, the following 

 brief notes concerning the reproduction in several of the groups of Cryptogams 

 have been inserted : - 



( 1 ) No sexual reproduction has yet been demonstrated in the very lowest 

 forms of vegetation. Such plants are termed Protophytes. The fungi which 

 are associated with fermentation and putrefaction, and certain of the simplest 

 alga?, are examples of the group. 



In the study of the Protophytes the beginner can examine with profit the 

 cells of common yeast. Care should be taken to distinguish between the cells 

 of the plant and the grains of starch with which compressed yeast is generally 

 associated. 



The simple one-celled plants with 

 chlorophyll which belong to this group 

 can be found in almost any stagnant 

 water. They are spherical, and are fre- 

 quently grouped in twos or fours. 



(2) The sexual process in Zygophytes 

 is characterized by the confluence of the 

 protoplasmic masses of two very similar 

 cells by which a new mass is formed 

 as the starting-point of the new indi- 

 vidual. In most of these zygophytes 

 there is no plain distinction of sex. 

 Some of the lower moulds and many 

 of the filamentous algse are examples 

 of the group. 



Excellent specimens for study may 

 be found in stagnant or slow-running 

 water in spring and through the sum- 

 mer. By careful search it is possible 

 to detect cases in which the process of 

 conjugation has advanced somewhat : 

 such specimens can be kept under ob- 

 servation by having the slide sufficiently 

 warm and constantly supplied with fresh 



water, when the different stages of conjugation and of cell-division may be 

 examined. 



FIG. 209. Spirogyra, illustrating the mode of fertilization in the Zygophytes. 

 Approximating cells of two filaments produce extensions which become conjoined ; the 

 protoplasmic masses in these cells become confluent, forming a single mass which after 

 escaping becomes clothed with a cell-wall and develops into a filamentous chain of 

 cells. In this case there is no appreciable distinction of sex. 



