PROCEEDINGS OF SOCIETIES. 51 



few instiinces only of the more striking differences can be given liere. The 

 pollen of the Cera7iiace(S and ('ampanulacece is for the most part globular, but 

 while some of the grains are quite smooth, others are covered with spines ; 

 thus the pollen of Campanula media has a number of short spines sparsely 

 scattered over the surface of the grain, but C. rapunculoides is wholly destitute 

 of them. In other plants these spines are replaced by tubercles, and both 

 spines and tubercles vary greatly in length and number ; for example, in Vale- 

 riana tuberosa the spines are only half the length of those on the pollen of V. 

 montana, the grains being also slightly smaller. The pollen of the LiliacecB is 

 often covered with a more or less prominent reticulation, which is subject to 

 much variation ; compare, for example, the coarse network which invests the 

 pollen oi L ilium croceum with the finer reticulation of L. Canadense, the grains 

 of the latter species being much more globose and smaller. 3. Dimensions. 

 Some instances of the diffei-ences observable in the size of pollen grains have 

 already been jjublished by Professor Gulliver, whose measurements of the 

 pollen of various species of Ranunculus * show the help that may be derived 

 from this character ; R. arcensis is nearly twice the size of R. Mrsutus, their 

 dimensions being respectively ^4y and -g^-g of an inch. I have not had the 

 time to make similar careful measurements with the micrometer, but I have 

 seen sufficient to be satisfied that while there is considerable variation in di- 

 mensions between the pollen of one species and that of another, they are 

 tolerably constant in size in the same species. For some noticeable differences 

 compare the smaller pollen of Epilobium hrachycarpum witli the larger pollen 

 of E. Fleischeri or that of Senecio Galliciis with 5. incanus, the spines on the 

 latter species being also much coarser. Again, the pollen of Silene acaulis is 

 but half the size of that of S. alpina, the latter having some beautiful markings 

 in addition ; the pollen grains of tliis genus differ from the usual caryophylla- 

 ceous type in not having the pits or depressions common in the Order, so that 

 the grains become spherical rather than polyhedral. 4. Colour. This is not 

 80 reliable a cliai'acter for differentiation as the others noticed, since species 

 differ amongst each other according to the soil, etc., of the place where they 

 have grown. I remember gathering, some years ago, near Ashbourne, Derby- 

 shire, a variety of Stellaria Holostea, having a dark purple pollen instead of 

 the ordinary pale yellow. An example or two under this head will suffice. 

 The pollen oi Ajuga Oenevensis is yellow, but that of A. pyramidalis is usually 

 white ; again, while the grains of Ornithogalum umbellatum are large and yellow, 

 those of 0. nutans are small and white. Some o'.ijection may be raised to any 

 reliance being placed upon the dry shrivelled- up grains of herbaria specimens 

 — such specimens being, in most cases, the only ones obtainable for purposes 

 of investigation ; but the structure of pollen is such as to bring into greater 

 prominence the pores, folds, valves, and other markings which are met with on 

 their surface after the grains have collapsed by tlie discharge of their contents. 

 In regard to the raoimting of tliese objects for the microscope, they show to 

 the best advantage when put up perfectly dry ; the cells sliould be sufficiently 



* See ' Journal of Botany,' Vol. IV. p. 281. 



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