1922] 



PFEIFFER — MONOGRAPH OF THE ISOETACEAE 87 



In early stages of leaf development, the ligule is longer than 

 the leaf, which is originally a low rudiment, soon developing 

 into an elongated structure with a broader base. The latter 

 develops more rapidly at first both in size and differentiation; 

 later the distal portion exhibits greater growth, resulting in the 

 elongated part above the ligule. In this region, there occur four 

 long air-channels, intersected at intervals by transverse par- 

 titions. The cavities are produced while the upper part of the 

 leaf is still meristematic. Groups of cells lose their contents, the 

 cells break apart through solution of the pectins of the middle 

 lamella, a process followed by disintegration of cells while inter- 

 vening patches fail to change and so form the septa. The con- 

 tinued growth of the living cells at the periphery brings about 

 increase in length and width of the cavities between these dia- 

 phragms. 



At the center of the leaf in the tissue between the channels, 

 the single collateral vascular bundle is located. Peripherally, in 

 the layers of green tissue, may occur strands of supporting 

 or mechanical tissue, which have commonly been called "pe- 

 ripheral bast." Since there seems to be no reason for assign- 

 ing a conducting function to the thick-walled cells in these 

 groups, the term peripheral strands is here substituted for bast. 

 By far the most common distribution of these strands is that in 

 which there is one at each side angle of the adaxial face and one 

 each at the adaxial and abaxial ends of the middle partition. 

 When six groups are present, the other two are at the ends of the 

 cross-partition. Any accessory strands produced are not usually 

 so well developed as these six. 



In leaves which are fertile, the sporangia occur singly at the 



base, on the inner face of the leaf. At maturity, each sporan- 

 gium appears to be fitted into an elongated or round cavity, 

 termed the "fovea" by Braun. From the margins except at the 

 base there may be a fine, one-celled layer of tissue, the velum, ex- 

 tending over the sporangium to a greater or less degree. Within 

 limits, the degree of development of this velum may be useful as 

 a diagnostic feature. Some forms characteristically show no 

 velum formation, as Isoetes M alinv erniana ; in /. Engelmanni, the 

 velum is usually narrow but evident; in /. Orcuttii, it is complete, 

 covering the sporangium all the way to the base. In the form 

 /. Braunii, the American ally of /. echinospora Dur. it varies 

 from 1/3 to 2/3, or occasionally even more extended. 



