360 



METAMORPHOSIS 



will afterwards produce male or female reproductive organs ; the sporophyte 

 produces megaspores in megasporangia and microspores in microsporangia, 

 the former female, the latter male. 



This stage in the historical development of the Pteridophyta brings us 

 nearer to the condition found in the Phanerogams. As a result of Hofmeister's 

 epoch-making investigations it is possible to compare the Gymnosperms in all 

 essential points with the Pteridophyta. We must, however, refer to the text- 

 books on botany or to the expositions of the subject given by Goebel in his 

 Organography of Plants for details of the comparison, since the plan of the 

 present course of lectures does not include a treatment of morphological relation- 

 ships such as these. We shall confine ourselves to a discussion of the Angio- 

 sperms, whose likeness to the Pteridophyta is not so apparent. Angiosperms 

 also develop sporangia on leaves which, like those of Pteridophyta, undergo 

 metamorphosis. These metamorphosed leaves and the sporangia which they 

 produce are collectively spoken of as the flower. The metamorphosis of floral- 

 leaves has been already referred to at p. 349, and it would appear that phylo- 

 genetically this metamorphosis has been intimately associated with the forma- 

 tion of sporangia, but whether the same is true ontogenetically in each 

 individual case is more than doubtful. Floral-leaves, at all events, have not 

 lost their power of becoming foliage-leaves, and after the application of appro- 

 priate stimuli flowers may be induced to take on a green colour. 



This question, however, need not be considered here, for it is the sporangium 

 and not the leaf from which it arises that is more especially before us. We 



may distinguish microsporangia (pollen-sacs) developed 

 on staminal leaves and containing microspores (pollen- 

 grains), and megasporangia (ovules) enclosed within 

 carpels, each containing, for the most part, one mega- 

 spore or embryo-sac. Very frequently we find micro- 

 and mega-sporangia united in the same flower, and their 

 mode of occurrence is determined exclusively by internal 

 factors, acting in such a way that the microsporangia are 

 developed first, the megasporangia later. 



The case where the flowers are unisexual, and where 



the sexes are distributed on different plants, requires 



further investigation as to the factors which bring about this determination of 



sex, but no definite conclusions on the matter have as yet been reached (Stras- 



BURGER, 1900). 



Let us now trace the further development of the spores. The microspore, 

 without any further growth, becomes transformed into a prothallus, dividing 

 into two cells of unequal size (Fig. 109). The small cell becomes the 

 antheridium, dividing later into two male cells, the large cell remains sterile and 

 has a special function to perform. This function becomes evident so soon as 

 the microspore reaches the stigma through the agency of wind currents or insects. 

 So soon as it has become securely fixed in that situation it proceeds to develop 

 a long hypha-like pollen-tube, which (Fig. no) forces its way deep into the 

 megasporangium, breaks through the wall of the megaspore, and sheds into 

 it the two sperms. The earlier changes which take place in the megaspore are 

 very remarkable. The megaspore remains imbedded in the tissue of the spo- 

 rangium, hence the necessity for the formation of a pollen-tube. Its nucleus 

 undergoes three successive divisions, so that finally eight nuclei are present in 

 the megaspore, and these are arranged in a definite manner (Fig. no). Three 

 of them are situated at the end of the megaspore where the pollen-tube enters, 

 forming there what is termed the egg-apparatus {ie. Fig. no ; compare 

 Fig. Ill), each becoming surrounded by a layer of protoplasm obtained from the 

 mother-cell. The egg-apparatus thus consists of three naked cells. Similarly, 

 at the other end of the megaspore, three other cells [an. Fig. no) are formed, the 



Fig. 109. Tradescatitia 

 tiirginica. Grain of pollen. 

 Below, the antheridial, above, 

 the veeetative-cell. x 540. 

 From the Bonn Textbook. 



