42 THE MORPHOLOGY OF PTERIDOPHYTES 



marked endodermis. The cortex is again divisible into three 

 regions, the innermost containing phlobaphene, the middle 

 region being heavily lignified and the outermost being 

 photosynthetic. The chlorophyllous cells in this outermost 

 region are elongated and irregularly 'sausage shaped', with 

 abundant air spaces between them, which connect with the 

 stomata in the epidermis. The leaves are arranged in a 

 roughly spiral manner in which the angle of divergence is 

 represented by the fraction i, but although internally they 

 are composed of chlorophyllous cells like those of the outer 

 cortex of the stem, they can contribute little to the nutrition 

 of the plant, for they are without stomata as well as having 

 no vascular supply. 



In this last respect, the leaves are in marked contrast to 

 the fertile appendages, for these each receive a vascular 

 bundle, which extends to the base of the fused sporangia, or 

 even between them. In their ontogeny, too, they are markedly 

 different from the leaves, for they grow by means of an 

 apical cell, whereas the young leaf grows by means of 

 meristematic activity at its base.^^ Shortly after the two 

 leaves have been produced on its abaxial side, the apex of 

 the fertile appendage ceases to grow and three sporangial 

 primordia appear. Each arises as a result of perichnal 

 divisions in a group of superficial cells, the outermost 

 daughter cells giving rise, by further divisions, to the wall of 

 the sporangium, which may be as much as five cells thick at 

 maturity. The inner daughter cells provide the primary 

 archesporial areas, whose further divisions result in a mass of 

 small cells with dense contents. Some of these disintegrate 

 to form a semi-fluid tapetum, in which are scattered groups 

 of spore-mother cells, whose further division by meiosis 

 gives rise to tetrads of cutinized spores. 



The genus Tmesipteris is much more restricted in its 

 distribution than Psilotum, for T. tannensis is known only 

 from New Zealand, Australia, Tasmania and the Polynesian 



