POLLEN-GRAINS. — OVULES. 429 



the inner coat of their cell, insinuating themselves between the 

 loosely-packed cells of the Stigma, grow downwards through the 

 Style, sometimes even to the length of several inches, until they 

 reach the Ovarium. The first change, — namely, the protrusion of 

 the inner membrane through the pores of the exterior, — may be 

 made to take-place artificially, by moistening the Pollen with water, 

 thin syrup, or dilute acids (different kinds of Pollen-Grains requir- 

 ing different modes of treatment) ; but the subsequent extension 

 by growth will only take-place under the natural conditions. 



319. The darker kinds of Pollen may be best mounted for the 

 Microscope in Canada balsam ; but as this renders the more 

 transparent kinds too faintly distinguishable, it is better to mount 

 them either dry or (if they will bear it without rupturing) in 

 fluid. The most delicate and interesting forms are found, for the 

 most part, in plants of the Natural families Amarantacece, Cicho- 

 racece, Cucurbitacece, Malvacece, and Passifiorece j others are fur- 

 nished also by Convolvulus, Campanula, Oenothera, Pelargonium 

 (Geranium), Polygonum, Seclum, and many other Plants. It is 

 frequently preferable to lay-down the entire Anther with its adhe- 

 rent Pollen-Grains (where these are of a kind that hold to it), as 

 an opaque object ; this may be done with great advantage in the 

 case of the common Mallow {Malta sylvestris) or of the Hollyhock 

 (Althaea rosea) ; the Anthers being picked soon after they have 

 opened, whilst a large proportion of their Pollen is yet undis- 

 charged ; and before they have begun to wither, being laid down 

 as flat as possible between two pieces of smooth Blotting-Paper, 

 then subjected to moderate pressure, and finally mounted upon a 

 black surface. They are then, when properly illuminated, most 

 beautiful objects for Objectives of 2-3rds, 1, 1^, or 2 in. focus, 

 especially with the Binocular Microscope. 



320. The structure and development of the Ovules that are pro- 

 duced within the Ovarium at the base of the Pistil, and the opera- 

 tion in which their Fertilization essentially consists, are subjects of 

 investigation which have a peculiar interest for scientific Botanists, 

 but which, in consequence of the special difficulties that attend the 

 inquiry, are not commonly regarded as within the province of 

 amateur Microscopists. — The Ovule, in its earliest condition, is, 

 like the Anther, a mass of cells in which no part is differentiated 

 from the rest ; gradually this body, which is termed the Nucleus, 

 is found to be enveloped in one, two, or three coats, which are 

 formed by the multiplication of cells that at first constitute merely 

 an annular enlargement at its base ; these coats, however, do not 

 entirely close-in around the nucleus, at the point of which there 

 always remains a small aperture called the Micropyle. In the 

 interior of the nucleus a large cavity is formed, apparently by the 

 enlargement of one of its cells at the expense of those which sur- 

 round it ; and this cavity, which is called the Embryo-sac, is at 

 first filled only with a liquid Protoplasm. Some little time before 



