118 BOTANY. [chap. IV. 



single cell that owes its important properties to the 

 mingling of the whole or a portion of the protoplasm of 

 two distinct cells; this mingling of protoplasm consti- 

 tutes the act of fertilization. If the two cells concerned 

 in fertilization are produced by the same plant, self- 

 fertilization is the result ; if produced by distinct plants 

 of the same kind, cross-fertilization is effected. 



Sexual reproduction is shadowed in amongst the 

 primitive types of Algse, under the form of motile 

 portions of protoplasm that escape from the cells and 

 swim about in the water by means of very delicate cilia 

 or prolongations of their protoplasmic substance ; such 

 cells are not furnished with a cell-wall, and after swim- 

 ming about for some time, approach each other in pairs, 

 which eventually blend together, withdraw their cilia, 

 and form a cell that soon secretes a cell-wall. This cell 

 either at once, or more frequently after a period of. rest, 

 germinates and gives origin to the vegetative condition 

 of a new individual. This, the most primitive condition 

 of sexual reproduction, is known as conjugation. In 

 many cases the two conjugating cells are similar in 

 shape and size, and externally present no functional 

 differences ; in other forms one, presumably the male 

 element, is smaller than the other, the female ; but in 

 many instances bodies that usually conjugate become 

 inclosed in a cell-wall without doing so, and at the 

 proper time germinate and produce plants agreeing in 

 every respect with those originating from cells result- 

 ing from conjugation, proving that the sexual method 

 is not indispensable, but rather in the first phase of 

 evolution. Eeproduction by conjugation occurs only 

 in certain groups of the Algae and Fungi, and in the 



