2 LECTURE I. 



on account of its universal presence in living organisms, it 

 has been described as the physical basis of life. It usually 

 occurs, as in these cases, in the form of minute individualised 

 masses, each such mass being termed a cell. The membrane 

 which surrounds the protoplasm of the larger zoospores is 

 the cell-wall, and it may be shewn by appropriate tests that 

 it consists of a substance known as cellulose: an investing 

 membrane of this kind is present in the great majority of 

 plant-cells, its presence is, in fact, the general rule, but it is 

 not absolutely essential. The essential part of a living cell is 

 its protoplasm, the cell-wall being a secondary formation, a 

 product of the vital activity of the protoplasm. This relation 

 between protoplasm and cell-wall can be well made out by 

 observing the subsequent changes which these zoospores 

 undergo. After moving actively for a time they come to 

 rest, lose their cilia, acquire a spherical form, and the proto- 

 plasm surrounds itself with a firm cellulose membrane, 

 each such non-motile cell constituting a Haematococcus plant 

 (Fig. I. B). After a time, if the external conditions are 

 favourable, the protoplasm divides into a larger or smaller 

 number of segments, the cell-wall is ruptured, and the seg- 

 ments of protoplasm are set free as actively moving ciliated 

 cells (zoospores), destitute of a cell-wall, that is, as primordial 

 cells. It is during their motile period that the larger zoospores 

 clothe themselves with a cell-wall as described above. 



In considering the life-history of Hsematococcus we ob- 

 serve that it maintains itself, that it reproduces its kind, and 

 that at one period of its life it is endowed with the power 

 of active motion ; phenomena which necessarily imply that 

 the organism is constantly obtaining supplies of matter and 

 of energy from without. The organism exhibits these phe- 

 nomena, or rather it performs certain functions of which the 

 phenomena are the outward and visible sign, in virtue of 

 certain fundamental properties with which its protoplasm is 

 endowed, properties which are possessed likewise, some in a 

 higher some in a lower degree, by the protoplasm of all living 

 plant-cells. It is necessary therefore, in commencing the 

 study of the Physiology of Plants, the study, that is, of their 



