326 THE BIOLOGY OF FLOWERING PLANTS 



in a peculiar covering and well stocked with food. It is 

 capable of dispersal and also of lying dormant and resisting 

 desiccation. 



We may look on the seed habit as the logical conclusion 

 (for the time being) of an evolutionary trend shown by plant 

 life on the land surface, a trend which was initiated by the 

 reduction of the gametophytic generation. Perfect protec- 

 tion of the female gametophyte and independence from the 

 necessity of external water are possible only when the 

 gametophyte is enclosed in the tissues of the parent. In 

 early stages the megaspore was not enclosed, but its retention 

 in the sporangium, and the prolonged attachment of the 

 sporangium itself gave further advantages in the way of 

 nourishing the ovum. The typical properties of the spore 

 were lost and the functions of dispersal and dormancy were 

 transferred to the new organ, the seed. To begin with, 

 the seed was cast loose after fertilisation, but before the 

 development of the embryo. A longer attachment to the 

 parent organism, however, brought with it the possibility 

 of better protection and nourishment of the embryo, and 

 of its provision with a food store, and so the modern seed 

 has been evolved. 



§ 4. Parthenogenesis and Apogamy 



The process of reduction of the gametophyte has not 

 stopped at the stage we have indicated as being character- 

 istic of the angiosperms. Many cases are now known in 

 which an embryo is developed without fertilisation having 

 taken place. This happens regularly in many plants, but 

 whether it is merely an abnormality, or whether it is a 

 comparatively new evolutionary departure in the angio- 

 sperms, we do not precisely know. 



Three different types exist. 



I . Parthenogenesis is the development of an unfertilised 

 ovum. It is known in the Compositas, e.g. in Antennaria 

 and Hieracium, in the Rosaceas, e.g. in Alchemilla, in the 

 Ranunculaceas, e.g. in Thalictrum, and in some other 



