358 MORPHOLOGY. 



certain places, either like slits or in well-marked circles, where it is 

 either entirely wanting, or becomes so exceedingly thin as to escape 

 the eye. The number and arrangement of these places is very 

 different ; thus the generality of Monocotyledonous pollen granules 

 have only a longitudinal slit (as in Lilium) ; some Dicotyledons 

 have very many (as Poly gala) \ the generality of Dicotyledonous 

 pollen granules have three circles uniformly distributed at their 

 equators (as in Centaur ecs), or four placed towards the angles of the 

 tetrahedron, or they have a large number (as Polemonium cmruleum 

 and Ipom&a purpurea). These openings are not always free, but 

 sometimes covered by a lid-like portion of the secreted layer, which 

 is entirely separated from the rest of the membrane (as in Pepo). 



The contents of the pollen-cells are originally almost purely 

 granular, with a small quantity of fluid; by degrees, however, the 

 granules are dissolved, and the thin contents become watery and 

 almost transparent, whilst the granules, still remaining undissolved, 

 appear as globules of mucilage. Towards the time of the matura- 

 tion of the pollen granules, these increase in size, and other small 

 granules appear amongst them of some undetermined substance, 

 coloured yellow by iodine (Inuline ?), and minute globules of oil ; 

 very frequently, also, starch granules, in greater or smaller num- 

 ber, sometimes of peculiar form (e. g., in the Onagrace&\ and always 

 differing much in size in the same pollen granule. By their presence 

 the fluid becomes more concentrated, losing water and acquiring an 

 extraordinary endosmotic power, even towards acids, on the appli- 

 cation of which it expands, the pollen-cell bursts, and its contents, 

 being protruded, coagulate into the form of an intestine. The 

 pollen granule, which towards the close of its development is very 

 much expanded, contracts again a little when quite mature, on 

 account of the loss of the water, and forms considerable folds in 

 the direction of the slits or pores, which are again effaced by the 

 absorption of water. The movement of the contents in reticularly- 

 connected currents has ceased in all the mature pollen granules 

 (with the single exception, as yet known, of the long cylindrical 

 pollen granule of Zostera marina) ; but instead of this, the various 

 granular contents of the pollen-cell exhibit active molecular motion : 

 this is often seen while the contents are still within the pollen-cell, 

 and always after they are expelled, even in the pollen of old speci- 

 mens in herbaria, and after the operation of tincture of iodine. 



We have received from Fritsche, in his work on the Pollen, published 

 in St. Petersburg!! in 1837, a collection of most careful and accurate in- 

 vestigations respecting the condition of the external pollen membrane in 

 its perfect state, to which I must here refer. In some plants he distin- 



fuished three several layers in the outer membrane of the pollen granule, 

 do not think myself obliged to accept his terminology, since it is not 

 in accordance with the most recent investigations. Here the pollen-cell 

 arid the secreted layer are opposed to each other ; this last may be single, 

 or it may be divided into three layers, but it is always opposed to the 

 pollen-cell merely as a single whole. The individual layers, however, 



