VEGETABLE PHYSIOLOGY 



CHAPTEE 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 sub- 

 stance itself, and various constituents of the body which 

 have been constructed 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 greatest in amount. 



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, 



FIG. 1. ZOOSPOBE OF 

 Ulothrix. x 500. 



