71 



and so is four-sided. It is best described as a tetra- 

 hedral apical cell. The three sides of the triangle are 

 embedded in the surroundiDg tissue and the base 

 forms the free upper surface. 



In the case of stems, segments are cut off in 

 regular alternation parallel to the three sides em- 

 bedded in the tissue. These divide up before settling- 

 down permanently. 



In the case of roots the division is the same, and 

 the free upper surface cuts off segments which go to 

 form the cap cells. 



SPOROPHYTE OP ASEXUAL GENEPATION. 



The ReproductiYe Organs arise on the under 

 surfaces of the leaves in groups called sori. Each 

 sorus is covered with an epidermal outgrowth called 

 the Indusrum, which is shield-shaped. Each sorus 

 consists of many sporangia. The placentae arise as 

 little swellings above the vascular bundles. Each 

 sporangium arises from one superficial cell. When 

 mature a sporangium consists of : — 



1. A short stalk which is branched. The 

 terminal cell of the branch is large, and filled 

 with mucilage. 



2. A globe-like head, consisting of 



a. An outer wall one cell thick. 



b. Two tapetal layers cut off from the arche- 

 sporium. 



c. Sixteen spore mother cells. 



