Lecture XXVI. 237 



Pine. In the megasporophylls the divergence is still greater. The 

 closed ovary is practically a new organ furnishing special protection 

 to the megasporangium, but involving other changes, viz. the 

 formation of the receptive stigma, the nutritive and guiding style 

 and the prolonged growth of the pollen-tube (male gametophyte). 

 The megasporophyll is still evidently in its origin a leaf, but the 

 approximation of its margins over its upper surface and their 

 fusion changes the flat leaf into a closed capsule. This change 

 may be followed in the development of each megasporophyll 

 (carpel), and it is only in the final stages of development that the 

 closing of the ovary is completed, and the condition, characteristic 

 of the highest group of seed plants, viz. the inclusion of the ovules 

 and seeds in closed carpels, is attained. Hence it is that this 

 group is called the Angiosperms in contradistinction to those 

 which, like the Pine, have the ovules and seeds exposed and are 

 styled the Gymnosperms. 



The development of special attractions for insects in connection 

 with the spore producing axis or flower is a modification of far- 

 reaching importance, and being combined with a suitable disposi- 

 tion of parts evidently secures the direct transference of the 

 microspores (pollen) to a position which will ensure fertilisation. 

 This avoids the wasteful production of microspores, which is 

 necessary when the arrival of the microspores into proximity 

 with the megaspores is fortuitous, as in the case of the Pine. 

 Thus it has been found that the bright colours and wonderfully 

 diverse shapes and mechanisms of the flowers of the Angiosperms 

 have direct relation with this insect-pollination and are in the vast 

 majority of cases only found in those which are dependent on 

 insects for the transference of their pollen to the stigma. On the 

 other hand, where the wind is the agent for pollination inconspicu- 

 ous flowers and different mechanisms prevail such as are found 

 generally in the Gymnosperms and also in certain of the Angio- 

 sperms. 



The increased complexity of the sporophyte of the Pine is 

 associated with a reduction and simplification of the gametophyte. 

 In the Buttercup the reduction of the gametophytes has gone a 

 step further. The microgametophyte or male gametophyte at its 

 maximum development is a minute ccenocytic hypha, containing 

 three nuclei, two of which are the nuclei of the sperm-cells. No 

 vegetative tissues are developed and no cellular sexual organs. 

 Such a complete absence of differentiation is rendered possible by 

 its holozoic nutrition and its parasitic habit. It is remarkable that 

 in spite of its dimensions being greater than those of the micro- 



