112 THE MORPHOLOGY OF PTERIDOPHYTES 



thick, of which the outer layer shows a characteristic spiral 

 thickening. 



Each spore, as it matures, has deposited round it four 

 spathulate bands, which are free from the spore wall except 

 at a common point of attachment (Fig. i6N). These are 

 hygroscopic, coiling and uncoihng with changes in humidity, 

 and are referred to as 'elaters', although what function they 

 perform during dehiscence of the sporangium is not clear. 

 McClean and Ivimey-Cook^^ have shown that a distribution 

 curve of the size of spores in Equisetum arvense is a bi-modal 

 one, suggesting that a shght degree of heterospory exists, the 

 large spores being some 25 per cent larger than the small 

 ones. Furthermore, the smaller spores give rise to small male 

 prothalli, whereas the large spores produce hermaphrodite 

 ones. Whether this represents the early stage of the evolution 

 of heterospory, or the last stages of a reversion to homo- 

 spory, cannot be determined but, in any case, Uttle out- 

 breeding advantage is likely to accrue, because the elaters 

 become so entangled as the spores are being released that 

 they are usually distributed in groups. Reports of com- 

 pletely dioecious prothalh have been pubHshed, but these are 

 probably based on observations made at a single moment in 

 time. The prothalh o^ Equisetum are long lived, and extended 

 observations would probably show that any one prothallus 

 has archegonia alone for a time and then antheridia alone, 

 as has indeed been demonstrated in some species. Differences 

 in nutrition can also influence the behaviour of the prothalh 

 for, under favourable conditions, only male prothahi result. 

 Further work on this fascinating subject is clearly necessary. 



The prothallus consists of a flat cushion of tissue, varying 

 in size from i mm across to 3 cm in some tropical species. 

 From the underside are produced abundant rhizoids, and 

 from the upper side numerous irregular upright plates, or 

 lobes, which are dark green and photosynthetic. Archegonia 

 are formed in the tissue of the cushion between the aerial 

 plates and ji& ve projecting necks of two or three tiers of 



,.^^ 



