XV.] THE BRACKEN FERN. 449 



The nutrition of the Fern takes place essentially as in 

 the other chlorophyll-containing plants previously con- 

 sidered. The root-hairs constitute the organs for the ab- 

 sorption of water and dissolved salts. As regards the 

 vascular bundles, the xylem has been found to be the tissue 

 in which the water is conducted to the leaves, while the 

 phloem serves for the transference of assimilated food, 

 especially proteids, from the leaves to all parts of the plant. 



Passing on to the reproductive organs, the sporangia have 

 first to be considered. The study of their development has 

 shown that each sporangium arises from the growth and 

 subdivision of a single epidermal cell of the leaf. The 

 sporangium consists of the pouch-shaped head above de- 

 scribed, which is borne on a short stalk. The interior of 

 the former part is occupied by the spores, usually 64 in 

 number, which are formed by the division into four of six- 

 teen mother-cells. The thick rim of the sporangium (called 

 the annulus)' consists of a row of thick- walled cells ex- 

 tending from the stalk over the top of the sporangium. The 

 annulus contracts in dry weather and thus tears the spo- 

 rangium open, setting the spores free. Each of the latter is 

 a single cell, and the outer layer of its wall is strongly cuti- 

 cularized. 



When the spores are sown upon damp earth, or a tile, or 

 a slip of glass, and kept thoroughly moist and warm, they 

 germinate. Each spore bursts its outer wall and gives rise 

 to a tubular filamentous prolongation, the protoplasm of 

 which contains chlorophyll-grains. At the base of this a 

 similar but colourless process, the first root-hair, is de- 

 veloped. The green filament at first undergoes transverse 

 division, so that it becomes converted into a series of cells. 

 Then, the cells at its free end divide longitudinally, as well 

 as transversely, and thus give rise to a flat expansion, which 

 M. 29 



