LECTURE XX. 



So far we have considered the outstanding external features and 

 the most important points in the gross and minute anatomy of 

 Aspidium, and we have seen in a general way how the construction 

 and the arrangement of its parts meet the requirements of the 

 plant for nutrition, respiration, and 

 growth. It now remains to examine 

 the outlines of its life-history and its 

 methods of reproduction. 



During the summer leaves are un- 

 folded which, while they resemble in 

 shape and structure the foliage-leaves 

 formed early in the year, present this 

 difference that the upper pinnae are 

 studded over on the under surface 

 with numbers of round, brown patches, 

 about a couple of millimetres in dia- 

 meter. Examined with a lens each 

 patch is seen to be composed of a 

 heap of rounded bodies. Each heap 

 is called a sorus. There are some five 

 or six sori on each pinnule. The sorus 

 is covered over by a membranous scale, 

 the indusium, which is circular in 

 outline with a deep indentation on one 

 side. The indusium is held in posi- 

 tion by a short stalk or column at- 

 tached to the under surface of the leaf 

 and rising up from the midst of the 

 sorus. The bodies forming the sorus 

 are sporangia. Each has the form of 

 a biconvex lens and it is attached 



to the leaf by a slender stalk. The diameter of a sporangium is 

 about 0-2 mm., and the length of its stalk is somewhat more than 

 its diameter. The sori are found on the surface of the leaf im- 

 mediately below a conducting tract. Here the tissue of the leaf 



161 II 



FIG. 39. Aspidium filix-mas, 

 pinna of leaf. 5, sorus. 

 (From Evans' An Inter- 

 mediate Textbook of Bot- 

 any.) 



