Plants, and Parthenogenesis. 97 



some of which I figured in my ' Flora Columbiae ' (I beg to refer 

 particularly to plates 52 and 57). The endogenous c&\h, which 

 here clothe the extine as a completely closed membranous layer, 

 are in these spores, as also in almost all others, not entirely 

 flattened, but resemble the " dot-cells " (the intermediate cor- 

 puscles of Fritsche), and are, in the groups named, polyhedral, 

 and in many others globular in form. When they contain no 

 younger generation of cells, it is so much the more a problem 

 whether they belong to the series of " dot-cells " or of " pore- 

 vesicles ; " but this is a matter of no consequence for the under- 

 standing of their anatomical structure. Those spores enveloped 

 by cells are particularly interesting on account of the subsequent 

 development of a sort of setse, which, as their age increases, cover 

 their surface, and entirely differ from those which derive their 

 origin from a peripheral elongation of the poi'ous vesicles. Further 

 development takes place only in those walls of the cells forming 

 the outer surface of the spores which lie next the intine and are in 

 contact with each other (being in this respect somewhat similar to 

 the cells of the sporangial ring and to the parenchyaia-cells enve- 

 lopingthevascularbundleoftheFern), — mostly, indeed, only where 

 tlie three cells touch ; for they are not completely in apposition. 

 The extine itself and the superposed peripheral wall of endogenous 

 cells are not thickened, but on the contrary are destroyed, in the 

 older spores, so that only the thickened angles and walls are left 

 behind, setiform and spike-like. 



Similar structures are also found among the pollen of Phane- 

 rogamia, i. e. in Cobcea scandens, as Mohl has tolerably well 

 pointed out. On the contrary, Fritsche and Schacht question 

 whether the cellular reticulated envelope of the exterior of these 

 pollen-grains is composed of actual cells ; for, according to their 

 views respecting the growth of the cell-wall, this external wall 

 originates from the exudation and deposit of matters which, from 

 some inconceivable reason, take on this wonderful reticulation. 

 The study of the progressive development of these pollen-grains 

 at once overturns this fantastic idea, and renders it easy to 

 understand that the cells which are formed between the extine 

 and intine, and construct a complete layer between the two, 

 become lignified in the same manner as has been described in 

 the spores of the Polypodiacese ; only that, in addition, those cells 

 in mutual contact become thickened and porous, and combine 

 to form a series of radiating pores. 



This cellular external tunic of Fern-spores often breaks up into 

 its three component portions, which are divided from each other 

 by stronger, thicker walls ; and from all this we gather that the 

 cellular envelope of the smooth intine of the Ferns is composed 

 of three cells coating the very delicate, transient extine of the 



Ann. £f Mag. N. Hist. Ser. 3. Vol. Wii. 7 



