310 



part globular, but while^some of the grains are quite smootli 

 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 G. rapuncidoides is wholly destitute of 

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

 and both spines and tubercles A^ary greatly in length and number; 

 for example, in Valeriana tiiherosa 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 Liliacece is often covered with 

 a more or less prominent reticulation, which is subject to much 

 variation ; compare, for example, the coarse network which in- 

 vests the pollen of Lilium 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 differences observable 

 in the size of pollen grains have already been published by Pro- 

 fessor Gulliver, whose measurements of the pollen of various spe- 

 cies of Ranunculus show the help that may be derived from this 

 character ; R. arvensis is nearly twice the size of it!, hirsutus, 

 their dimensions being respectively 4-yo^l^ ^^^ stttt^^^ o^ ^^ 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 dimensions 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 

 oi Epilohium hrachycarpum with the larger pollen of E. Fleischeri 

 or that of Senecio galliciis with S. incamis, 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. alfina, the latter having 

 some beautiful markings in addition ; the pollen grains of this 

 genus difter from tlie usual caryophyllaceous type in not having 

 the pits or depressions common in the order, so tliat the grains 

 become spherical rather than polyhedral. 



4. CoLouB. This is not so reliable a character for differentia- 

 tion as the others noticed, since species differ amongst each other 

 according to the soil, &c., of the place where they have grown. I 

 remember gathering some years ago, near Ashbourne, Derbyshire, 

 a variety of Stellaria Solostea having a dark purple pollen instead 

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

 will suffice. 



The pollen of Ajuga cjenevensis is yellow, but that of A. pyra- 

 midalis is usually white ; agaiu, while the grains of Ornitliogalum 

 umbellatum are large and yellow, those of O. nutans are small and 

 white. 



Some objection 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 pur- 

 poses of investigation ; but the structure of pollen is such as to 

 bring into greater prominence the pores, folds, valves, and other 



