20 



Prof. F. 0. Bower. Studies in the [Feb. 10, 



rounding the sporogencms mass ; it is, however, to be carefully dis- 

 tinguished from certain cells of the sporogenous mass, which also j 

 undergo an early disorganisation ; for about one-third of the cells of 

 the sporogenous mass do not form spores, but serve physiologically as 

 a diffused tapetum, yielding up their substance to nourish the other 

 young developing spores. This is another form in which sterilis- 

 ation of sporogenous tissue may appear. A similar arrest of some of 

 the sporogenous cells is found also in the Psilotaceae. 



I have already suggested a theory (loc. cit., p. 273) of the mode of 

 origin of the whole strobilus of Equisetum from a sporogonial head, 

 and have no reason to alter my opinion on this point. 



The synangia of the Psilotaceae have given rise to voluminous dis- 

 cussions ; one view, which is now very widely adopted, as to the 

 morphology of the parts which bear the synangia of these plants is, i 

 that the synangium is terminal on an abbreviated axis, which bears 

 in addition two foliage leaves. It will be seen that the investigation 

 of the internal details of development will support a simpler and more 

 probable explanation of the nature of these peculiar parts, that which 

 was indeed generally held by the older botanists, viz., that the whole 

 of each lateral appendage (sporangiopTiore) which bears the synangium 

 is a single leaf. This conclusion had already been arrived at by 

 Graf Solms, after examination of the external form of the developing 1 

 organs in Psilotum, for he found the synangium to arise below the 

 apex of the whole lateral appendage. No sufficient examination has, 

 however, yet been made of the internal details of the development 

 of these parts in the Psilotaceae, excepting a few observations by 

 Goebel on Psilotum ^'Bot. Zeit.,' 1881, p. 688), which were, however, 

 incomplete, through insufficiency of material. 



Tmesipteris being the genus with the simpler structure, it may be 

 described first. In their earliest stages of development, as lateral 

 outgrowths from the axis, the sporangiophores are not readily dis- 

 tinguishable from the foliage leaves in form or structure, while they 

 occupy a similar position upon the axis. In either case, a prismatic 

 or wedge-shaped cell occupies the apex, as seen in radial section, but 

 the tissues of the whole leaf are not readily referable to the seg- 

 mentation of a single initial. The first appearance of a synangium is 

 as an upgrowth of superficial cells of the adaxial face of the sporangio- 

 phore, immediately below its apex ; meanwhile the cells of the abaxial 

 side also grow strongly, while the apex itself does not grow so 

 rapidly ; so that the organic apex is soon sunk in a groove between 

 these stronger growths. The superficial cells which are to form the 

 synangium undergo periclinal and anticlinal divisions, to form about 

 four layers of cells ; all the cells of this tissue are at first very 

 similar to one another, but, later, two sporogenous masses become 

 differentiated ; they are not, however, clearly defined while young 



