POLLEN. 77 



The connectivum often appears under modified forms. Some 

 of these we have already noticed, as in the Sage. In the As- 

 clepias, also, the little horns observed in the flowers of these 

 plants are developments of the connectivun. In some cases it 

 is very much enlarged, in others forked, in others it forms a crest, 

 and again it forms a cup-like body articulated with the apex. 

 The position it occupies in these and other cases, will enable the 

 observer to determine to what organ it is to be referred. 



124. The pollen consists of exceedingly minute grains, which, 

 under the microscope, appear of various forms. In some they 

 are smooth and spherical, as in the Marvel of Peru ; in others 

 with conical papillae, as in the Hibiscus grandiflorus ; in others 

 angular, some nearly square and of every variety of geometrical 

 figure. It would be useless to specify the great variety of forms 

 under which this substance appears, as it has, as yet, been made 

 of little practical importance in arranging plants, although so far 

 as we have made observations on this subject, we believe it might 

 in some cases be made a good specific character, and in others 

 a generic one of much importance. We have never found a 

 variation of form in the same species ; and in some extensive 

 genera, so far as we have examined them, the form is inva- 

 riable. 



The student can scarcely find a field for more curious obser- 

 vation, if he has a good microscope, than is presented by the 

 pollen. The variety of beautiful forms it assumes, in different 

 species, and the curious structure of the pollen grain itself, present 

 subjects of much interest. 



12 J. The pollen grains inclose a fluid of molecular matter 

 essential to the production of the seed. The molecular forma- 

 tion may be beautifully observed by sprinkling some pollen on 

 the port-object of the microscope, and dropping on it some dilu- 

 ted sulphuric acid. The coats of some of the grains immediately 

 burst, and the contents of the grain are projected into the fluid, 

 and the molecules may be distinctly seen. The pollen has been 

 determined, by the most accurate observers, to consist of two 

 coats, at least; the outer and thicker one called the extijie, the 

 inner the intine, which is very extensible and exceedingly thin. 

 This may be exhibited by placing some pollen in very dilute 

 sulphuric acid ; instead of bursting, as in the case mentioned 

 in the preceding paragraph, projections will be seen to arise from 

 the surface of the grain and extend into the fluid. These lengthen 

 till the contents of the granule are exhausted, and consist of the 



124. Of what does the pollen consist ? What of the form and surface of 

 pollen grains I — 125. What do the pollen grains inclose ? llow may it b« 

 aeea ? What is the extine ? The intine ? 



