INTEODTJCTION. XI 



co-operating in carrying on the nutritive and reproductive 

 functions. It was also there proved (54-57), that " it is not 

 necessary for cell-development to be carried to any great extent 

 in order to constitute the fabric of a true and perfect plant ;" 

 on the contrary, the same laws of nutrition and reproduction 

 operate in the vital economy of plants composed of a few cells 

 as when vegetation is constructed on a scale of gigantic mag- 

 nitude and grandeur. In such plants, it is evident that we 

 have the phenomena of life existing under extremely simplified 

 conditions ; and if ever " man, the minister and interpreter of 

 nature," is destined to discover those morphological laws 

 which govern this evolution and endless repetition of the same 

 definite forms of vegetable and animal life from the same 

 embryos, it is here ihat he must commenc*e his investigations. 



Hitherto the attention of the student has been directed to 

 the consideration of cryptogamous vegetation, we are now 

 about to enter on the examination of vegetable life as unfolded 

 in the more complex and elaborate organization of the Phanero- 

 gamia, or flowering plants. 



In the lower forms of the Cryptogamia the essential organs 

 of vegetation, the root, stem and leaves, are blended together 

 into a flat or filamentous expansion of vegetable matter, 

 termed a thallus, from whence these plants have received 

 the name of thallophytes (6aM.6s a frond, ^vtbv a plant.) 

 These plants have no -vegetable axis or stem, and increase by 

 additions of matter to their periphery or circumference. They 

 have a tendency to grow in a horizontal rather than in a ver- 

 tical plane, their spores germinating indifferently in all direc- 

 tions from any part of their surface. 



The cells which constitute the tissue of thallophytes, in the 

 lower forms of their development, appear to retain the form, 



