PREFACE. 9 



and thus a flood of light was thrown on the organization of 

 plants. 



But how do the cells of plants and animals originate ? How 

 do they multiply and extend themselves so as to produce the 

 growth or enlargement of the organs ? These are difficult but 

 interesting questions, and botanical researches have enabled 

 us to reply to them satisfactorily. 



A German naturalist, Mohl, selected for observation one of 

 the fresh water algae, which had been previously figured and 

 described as Conferva glomerata. This simple, thread-like 

 plant was placed beneath the microscope, and the develop- 

 ment of the row of utricles of which its entire organization 

 consists, watched. Very soon, Mohl observed that the inte- 

 rior face of the cavity of one of the utricles presented towards 

 its middle part a fold, which increased almost imperceptibly 

 until it ended by forming a complete wall dividing the cavity 

 of the utricle into two parts. Each of these then dilated itself 

 into a new utricle. Thus in the place of one cell there were 

 two cells, which again divided in the same way, and so on. It 

 is in this way that a single cell gives rise to a row of connect- 

 ed cells, when the division takes place in one direction, and to 

 a plane or solid mass when it takes place in two or more di- 

 rections. There are other modes of increase which we shall 

 notice in the ensuing pages ; suffice it for the present to say, 

 that their discovery originated in the investigation of crypto- 

 gamous plants of extreme simplicity of organization. 



Up to this period it was believed .by the most eminent phy- 

 siologists, that animal and vegetable tissues differed widely in 

 their development, and that cells existed only in plants. Such 

 was the condition of things in 1838, when Schwann, taking up 



