INTRODUCTION. lili 



Some, and by far the greater part of, plants are propagated by produc- 

 tions called seeds, which are the result of the mutual action of the 

 stamina and pistils ; others are multiplied by bodies called sporules, of 

 the real nature of which little is yet known, further than that they do 

 not appear to result from the action of the stamina and pistils. Hence 

 plants are naturally and primarily divided into two great divisions, called 

 Phenogamous and Agamous. 



Physiologists have discovered that these peculiarities are connected with 

 others in anatomical structure of no less importance. For instance, plants 

 propagated by seeds, and possessing distinct stamina and pistils, have spiral 

 vessels ; while those which are increased by bodies not depending upon the 

 presence of these organs, are universally destitute of spiral vessels. To the 

 latter statement there is no known exception, — species to which spiral 

 vessels have been ascribed being found to possess nothing more nearly 

 related to those organs than ducts, or false tracheae. The former character 

 is not absolutely without exception ; the singular genus Rafflesia being 

 described both by Brown and Blume as without spiral vessels, Caulinia 

 fragilis not having them according to Amici, and Lemna being destitute 

 of them according to the evidence of others. But these exceptions are not 

 regarded of much importance. 



It therefore appears that two great divisions, established upon different 

 principles, agree in the kind of plants they comprehend ; Vasculares, 

 or those which have spiral vessels, being the same as Phenogamous 

 plants, and Cellulares, or those which have no spiral vessels, answering 

 to Agamous plants. 



Stamina and pistils being considered essential to a flower (no apparatus 

 whatever from which they are absent being understood to constitute one), 

 two other unexceptionable characters belong to these same divisions ; all 

 Vasculares, or Phenogamus plants, bear flowers, and all Cellulares, Aga- 

 mous plants, are flowerless. 



Two great but unequal divisions being thus established, upon both 

 anatomical and external characters, botanists have inquired whether simi- 

 lar differences of a secondary character could not be discovered among each 

 of them. Observations upon Cellulares have led to the establishment of 

 three groups of unequal importance, which are not, however, universally 

 received. Vasculares have been found to comprehend two great but 

 unequal tribes, differing essentially in the laws which govern their growth. 

 It has been ascertained that a large number of them grows by the addition 

 of successive layers of new matter to the outside, and that another, but 

 smaller number, increases by additions to the iuskb ; the youngest or 

 most newly formed parts being in the one case on the outside, and in the 

 other case in the inside. For this reason, one of these divisions has been 

 called Exogenous, and the other Endogenous. It is difficult to conceive 

 how the external increase of Exogenee could take place without some 

 adequate protection to the young newly formed tissue from the atmosphere 

 and accidental injury, and, accordingly, the substance called bark is created 

 by nature for that purpose, within which the new deposit takes place : as 

 this last is formed annually, the age of an Exogenous plant is indicated 

 in the trunk by imaginary lines called concentric circles, which are in fact 

 caused by the cessation of growth in one year, and the renewal of it in 

 another. The centre of this system is a cellular substance caHed pith. 



