CHAPTER XIX 



THE ANGIOSPERMAE : INTRODUCTION TO THE 

 HIGHER FLOWERING PLANTS 



The Angiospermae form the highest class of the Spermatophyta or Seed 

 Plants ; the Pteridospermae and Gymnospermae being the lower classes. 

 The name is derived from the two Greek words aggeion, a vessel, and sperma, 

 a seed, and it indicates the character which, above all else, separates them 

 from other Seed Plants ; namely, that the seeds are enclosed in an ovary, 

 which at maturity becomes a fruit. This is a morphological distinction, 

 but biologically they are further distinguished by the fact that the function 

 of pollination is taken away from the ovules and transferred to a special 

 receptive organ, developed by a portion of the ovary and known as the 

 stigma. 



The ovary is both morphologically and biologically a new organ. How 

 far it and other organs of the angiospermic flower can be compared with the 

 organs of lower plants we shall discuss when we deal with the Flower as a 

 structure. For the present we shall simply recognize that these floral organs, 

 whatever their origin, have become so developed and specialized in the 

 Angiosperms that new names must be used for them. 



An angiospermic flower may produce spores of only one kind or of both, 

 but the latter condition (hermaphrodite or amphisporangiate) occurs so 

 widely and so constantly in the majority of the families that it may be regarded 

 as characteristic, whereas in the Gymnosperms it is only an occasional anomaly. 

 Similarly, the transfer of pollen in Gymnosperms is normally by the wind 

 and only exceptionally by insects or other carriers. In Angiosperms insect 

 pollination is the rule, and indeed the adoption of this method has had so 

 profound an effect on the structure of the flower that it may be regarded as a 

 fundamental factor in the evolution of the class. 



Compared with the woody and long-lived strobili of Gymnosperms, 

 the angiospermic flowers are as a rule delicate and short-lived, and are 

 correspondingly more freely produced. The reproductive parts are usually 

 surrounded by an envelope of leafy structures, the perianth, which is often 

 highly coloured. 



The microsporangia or pollen sacs are united into anthers which are 

 borne on short stalks or filaments ; the two structures together form the 

 stamens. In the microspore there are no vestiges of a prothallus, only a 

 tube nucleus and a generative nucleus are present. 



The megasporangia are, as in Gymnosperms, enclosed by integuments, 

 usually two in number, to form ovules, which are enclosed within the ovary 



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