72 THE COMMENCEMENT OF THE PHANEROGAMIC FRUIT. 



Cryptogams lack blossoms, since these are not necessary for aquatic fertilization; 

 (3) that almost all Phanerogams, on the other hand, possess blossoms, since they 

 are required to protect and promote aerial fertilization. 



The very complicated structure of the parts immediately adjacent to the region 

 where the sexual protoplasts are developed depends upon the fact that fertilization 

 is aerial. The portions of protoplasm destined for fertilization can only be 

 adequately elaborated if their enveloping membranes are thin and delicate, and 

 suited for the osmotic transfer of materials. Such a membrane, however, is 

 incapable of protecting the protoplasm from the drying influence of the air; it 

 is absolutely essential that both the spermatoplasm and the ooplasm shall be 

 protected during the critical period by a suitable envelope. Thus one finds in 

 all Phanerogams — quite apart from the perianth — a protective mantle developed 

 around the sexual cells. This mantle has its cell -walls suitably thickened; its 

 outer layers afford the necessary resistance to desiccation, whilst deeper down an 

 ample supply of water is maintained. 



These characters are well shown in that constituent of the ovary from which 

 the seed will be ultimately produced. This portion is kno\ATi as the ovule. Every 

 ovule consists of a mass of tissue, the nucellus of the ovule in which the ooplasm 

 or egg-cell is concealed, and ah enveloping sheath, the integument, which may 

 be either single or double. Such ovules are borne in the genus Cycas (figs. 208^ 

 and 208 ^) without further covering than a fretwork of hairs which protects them 

 against too great drying up. In other Cycads and in the majority of Gymno- 

 sperms, of which the Cypress and Juniper, the Pine and the Fir, may be quoted 

 as examples, the leaf-like scales of the young fruit are so arranged that the 

 ovules produced on their surfaces are hidden from view and secure against 

 outside danger. In the other Phanerogams (the Angiosperms) the ovules are 

 concealed in a closed chamber — the pistil — the lower enlarged portion of which 

 is known as the ovary. 



In the construction of this chamber the chief part is taken by the floral axis 

 and by the floral-leaves known as carpels. So unequal, however, is the share 

 taken by these parts in the structure of the ovary that in some cases it is formed 

 almost entirely from the floral axis, and in others almost entirely from the 

 carpels. In consequence the apex of the floral axis, which is known as the 

 floral receptacle, shows an extraordinary variety of form. Thus in one series 

 of plants the receptacle is not excavated, but solid, assuming the form of a 

 knob, hemisphere, or cone (figs. 207^ and 207^); whilst in others it is concave 

 and excavated (figs. 208^ and 208 2). The forms met with in nature can be 

 produced artificially by taking a conical mass of soft wax and flattening its 

 summit, then gradually pressing it down into a saucer-like shape, and so on 

 until one has produced a hollow bowl. So in nature we have at one extreme 

 the solid cone, at the other the hollow vessel. Between these two extremes, 

 between the conical and excavated receptacles, we have the flat or disc-like 

 receptacle. It is hardly necessary to point out that in the growth and difier- 



