PILULARIA SPOROPHYTE 321 



a similarity to aquatic Phanerogams (compare p. 110, 

 171) and Equiseta. 



Cut also transverse sections of the rhizome of Marsilea : it 

 will be found to show a similar general plan of structure, but it 

 is more robust than Pilularia: sclerenchyma is found in the 

 cortex, and the xylem and phloem are more largely developed, 

 and better differentiated : the cylindrical form of the vascular 

 bundle corresponds to that of Pilularia, but the ring of vascular 

 tissue is here usually wider, and it will serve as a link to connect 

 these reduced bundle-systems with those of such as Aspidium 

 which have an open meshwork. 



III. Cut transverse sections of the leaf of Pihdaria : 

 and, treating as before, note the circular form of the 

 section : this is a most perfect centric type of leaf. 

 Examine and observe 



1. The epidermis, with numerous stomata. 



2. The mesophyll, consisting of a single hypodermal 

 layer, within which are large intercellular cavities 

 separated by radiating trabeculae, one layer of cells in 

 thickness, and partitioned by transverse diaphragms. 



3. A central, cylindrical, vascular bundle with a 

 definite endodermis : compare the structure of the 

 bundle with that of one of the bundles of the stem. 



IV. Cut transverse sections of the root, and observe 

 in them 



1. The epidermis, which in old roots is squeezed 

 out of shape. 



2. The cortex, which is composed of (a) an outer 

 hypodermal layer ; (&) radiating lamellae which separate 

 large intercellular spaces : single cells of these lamellae 

 occasionally grow into the cavities as large spirally 

 coiled internal hairs, which probably serve to 



y 



