114 BOTANY. 



consists of a central cell, surrounded by two outer layers of cells. From 

 the central cell a group of cells is formed by further divisions (Z>), which 

 finally become entirely separated from each other. The outer cells of the 

 spore case divide only by walls, at right angles to their outer surface, so 

 that the wall is never more than two cells thick. Later, the inner of these 

 two layers becomes disorganized, so that the central mass of cells floats 

 free in the cavity of the sporangium, which is now surrounded by but a 

 single layer of cells (E). 



Each of the central cells divides into four spores, precisely as in the 

 bryophytes. The young spores (G, H) are nearly colorless and are 

 tetrahedral (like a three-sided pyramid) in form. As they ripen, chloro- 

 phyll is formed in them, and some oil. The wall becomes differentiated 

 into three layers, the outer opaque and brown, the two inner more delicate 

 and colorless. 



Running around the outside of the ripe spore case is a single row of 

 cells (aw.), differing from the others in shape, and having their inner walls 

 thickened. Near the bottom, two (sometimes four) of these cells are 

 wider than the others, and their walls are more strongly thickened. It is 

 at this place (st. ) that the spore case opens. When the ripe sporangium 

 becomes dry, the ring of thickened cells (an.) contracts more strongly than 

 the others, and acts like a spring pulling the sporangium open and shaking 

 out the spores, which germinate readily under favorable conditions, and 

 form after a time the sexual plants (prothallia). 



The roots of the sporophyte arise in large numbers, the 

 youngest being always nearest the growing point of the stem 

 or larger roots (Fig. 67, C). The growing roots are pointed 

 at the end which is also light-colored, the older parts becoming 

 dark brown. A cross-section of the older portions shows a 

 dark-brown ground tissue with a central, light-colored, circular, 

 fibre-vascular bundle (Fig. 68, F). Growing from its outer 

 surface are numerous brown root hairs (r) . 



When magnified the walls of all the outer cells (epidermis and ground 

 tissue) are found to be dark-colored but not very thick, and the cells are 

 usually filled with starch. There is a bundle sheath of much-flattened cells 

 separating the fibro-vascular bundle from the ground tissue. The bundle 

 (Fig. 68, G) shows a band of tracheary tissue in the centre surrounded by 

 colorless cells, all about alike. 



