FLOWERLESS PLANTS 



153 



entire. The stem is underground. Large compound leaves (called /ron^/s) 

 are given off at mtervals along the stem. We call the underground stem a 

 rootstock. Try to find a rea- 

 son for calling it a stem rather 

 than a root. 



Note the arrangement of 

 the leaflets, or pinnce, of the 

 fern frond. Some of the pinnae 

 will show a series of little 

 brown dots on the under sur- 

 face. These structures, called 

 collectively the sori (singular 

 sorus), are made up of a num- 

 ber of tiny spore cases. These 

 spore cases, or sporangia, hold 

 the asexual spores. Exam- 

 ine a sorus with your hand 

 lens to make out its position 

 on the pinna with reference 

 to the veins. 



Mount a small bit of the 

 fern leaf which contains a 

 sorus under a very low power 

 of the compound microscope. 

 How are the sporangia ar- 

 ranged in the sorus ? 



If a single sporangium is 

 gently separated from the 

 mass and mounted carefully 

 in alcohol and water on a slide, 

 the following structures may 

 be found : (a) the stalk ; (5) a 

 thick wall row^ of cells, yellow 



in most specimens, which form the annulus, or ring; (c) the covering of 



thin-walled cells over the remainder of the 

 sporangium. A comparison of several spo- 

 rangia will bring out the fact that in an opened 

 sporangium the annulus is never broken. It 

 is always the thin-walled cells that are rup- 

 tured. The dark-colored spores may be seen 

 in the opened sporangia. 



Rock fern, polypody. Notice the underground stem 

 giving off roots {R) from its under surface, and 

 leaves (C) from the upper surface. .The com- 

 pound leaf or frond may bear sori (5) on the under 

 side of the leaflets. 



Section of sorus; s, sporangia; 

 i, indusium, or covering; b, 

 blade of the leaf. — Wossidlo. 



If fresh material is obtainable, it will be 

 possible to see how the spores get out of the 

 sporangium. A drop of glycerine run under the cover slip 

 of a slide holding a fresh unopened sporangium soon causes 

 the sporangium to snap open. If the sporangium is dry and 

 on the under surface of the fern leaf, the spores v.ill be scattered 

 for a considerable distance. An explanation for this snapping 

 open of the sporangium is found when we notice that the outer 

 walls of the cells forming the annulus are thinner than the inner 

 walls. This allows water to escape more rapidly on one side, 

 and pressure from without causes the cells to bend outward a sporangium 



