PSILOTOPSIDA 39 



much rarer plant, occurring in Jamaica, Mexico and a few 

 Pacific Islands, and is epiphytic with pendulous branches. 



The organs of attachment in both species are colourless 

 rhizomes which bear numerous rhizoidal hairs and which, 

 in the absence of true roots, function in their place as organs 

 of absorption. In this, they are probably aided by a mycor- 

 rhizal association with fungal hyphae, that gain access to the 

 cortex through the rhizoids. Apical growth takes place by 

 divisions of a single tetrahedral cell which is prominent 

 throughout the hfe of the rhizome, except when dichotomy 

 is occurring. It is said^^ that this follows upon injury to the 

 apical cell as the rhizome pushes its way through the soil 

 and that two new apical cells become organized in the 

 adjacent regions. In any case, there is no evidence of a 

 median division of the original apical cell into two equal 

 halves ; to this extent, therefore, the rhizome cannot be said 

 to show true dichotomy. 



In Psilotum nudum, some branches of the rhizome turn 

 upwards and develop into aerial shoots, commonly about 

 20 cm high, but as much as i m high in favourable habitats. 

 Except right at the base, these aerial axes are green and bear 

 minute appendages, usually described as 'leaves', despite the 

 fact that they are without a vascular bundle (cf. Psilophyton). 

 The axes branch in a regular dichotomous manner and the 

 distal regions are triangular in cross-section (Fig. 6A). In 

 the upper regions of the more vigorous shoots, the leaves 

 are replaced by fertile appendages (Fig. 6B) whose morpho- 

 logical nature has been the subject of much controversy. 

 Some have regarded them as bifid sporophylls, each bearing 

 a trilocular sporangium, but the interpretation favoured 

 here is that they are very short lateral branches, each bearing 

 two leaves and terminating in three fused sporangia. 



Psilotum flaccidum differs from P. nudum in two import- 

 ant respects : its aerial branches are flattened and there are 

 minute leaf-traces which, however, die out in the cortex 

 without entering the leaves (cf. Asteroxylon). 



