VEGETABLE PHYSIOLOGY 



CHAPTEK I 



THE GENERAL STRUCTURE OF PLANTS 



Examination of the body of every living organism shows us 

 that it is composed of different materials, which exhibit a 

 great deal of variety in the ways in which they are arranged. 

 These different materials fall very naturally into two classes, 

 which include respectively the living substance itself, and 

 various constituents of the body which have been con- 

 structed by it. The relative proportions in which these two 

 classes of materials exist vary very greatly in different 

 organisms ; in some of the simplest forms indeed we can 

 discern nothing structural except the living substance itself. 

 In others the materials constructed by the latter are much 

 the most conspicuous. 



When we study the life history of the simplest or the 

 most complex plant with which we can become acquainted, 

 we find that at some time or other in 

 its existence it is found in the form of 

 a minute portion of jelly-like material 

 which is endowed with life. Some- 

 times this piece of living substance is 

 motile, and can swim freely about in 

 water by means of certain thread-like 

 appendages which it possesses (fig. 1). 

 Such structures occur almost exclu- 

 sively among the lowest forms of plants, particularly the 

 seaweeds. They are known as zoospores, or zoogonidia, 



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