STRUCTURK.] SHELL OF POLLEN-GRAIN. 359 



(Taxus,) Juniperus, Cypress (Cupressus,) Arborvitse (Thuja,) 

 the outer membrane has, as Mohl states, so little extensi- 

 bility, that it is torn irregularly, and the inner membrane 

 protrudes beyond the crevices, and, swelling more and more, 

 generally disengages itself from the extine. It sometimes 

 happens that the inner membrane protrudes beyond the outer 

 shell, in the form of a short sac or tube : this phenomenon 

 may be produced artificially at will by placing the pollen- 

 grain in weak nitric or sulphuric acid; but it is quite a 

 distinct emission from that of the pollen-tubes hereafter to 

 be noticed. 



A third membrane, intermediate between the extine and 

 intine, was first noticed by Mohl in the pollen of the Yew 

 (Taxus), Juniperus, Cupressus, and Thuja. Fritzsche calls it 

 the Exintine, and finds it not only in these plants, but also in 

 Pinus, the Gourd, (Cucurbita Pepo), and Tigridia Pavonia, 

 and considers it probably a common structure. The same 

 minute observer speaks of four coatings to the pollen of 

 Clarkia elegans, calling the fourth, which is next the extine, 

 the Intexine; he also finds the same structure in other 

 Onagrads. 



Mohl names Asclepiads as those only in which pollen has 

 but one tunic ; but Fritzsche asserts that these plants have 

 both an extine and intine, and he figures them in Asclepias 

 syriaca ; he adds, that in Caulinia fragilis, Zannichellia pedun- 

 culata, Zoster a marina, and Naias minor, the pollen has really 

 nothing but the intine present. 



There are few forms of pollen in which the extine presents 

 the appearance of a vesicle completely closed. In many cases 

 the grains are marked by a longitudinal furrow on one side, 

 and look when dry like a grain of wheat. Mr. Griffith has 

 shown, as is above stated, that this appearance is caused by a 

 fissure in the extine, and if such pollen is put into water the 

 fissure becomes less visible. The presence of one or more 

 such clefts is of common occurrence ; and have been supposed 

 to be openings through the extine down to the intine ; Mohl, 

 however, considers them not to be really openings, but only 

 extremely thin spaces in the extine. Fritzsche calls them 

 pores, and regards them as certainly openings. Instead of 



