594 



FLOWER 



FLOWER 



which joins them, called the connective. This some- 

 times is extensive, and in a few plants is developed into 

 peculiar forms to aid in pollination, e. g., in salvias. 



The sporangia at maturity consist of two or four 

 (rarely more) layers of cells, constituting a wall, sur- 

 rounding a quantity of spores, the pollen. The inner 

 portion of the wall consists of a layer of 

 cells whose membranes are irregularly 

 thickened, usually in bands, so that in dry- 

 ing they warp the wall, rupturing it at the 

 weakest place. The lines of weakness are 

 usually definitely localized, so that each an- 

 ther breaks in a regular way. (a) The rup- 

 ture may run along the whole length of 

 the anther. In that case it commonly lies 

 at the junction of a pair of sporangia (the 

 left-hand groove in a, Fig. 831), which be- 

 come confluent, so that the dehiscent anther 

 may seem to have only two sporangia, when 

 it really has four. The pollen is thus 

 emptied out practically at once, though the 

 832 " Anther break may begin at the top and progress to 

 of 'Azalea, the base. Examples: lilies, grasses. (&) The 

 Showing de- slit ma > T be ver y short and S a P e widely, so 

 hiscence that a pore is formed through which the pol- 

 by pores, len is gradually sifted ( Fig. 832 ) . Examples : 

 the heaths, (c) In some plants the line of 

 breakage is curved, and the flap, so re- 

 leased, bends outward on drying, lifting 

 like a hinged lid, and closing again in 

 dampness. Examples: Mahonia, barberry, 

 cinnamon. 



The pollen spores are, at maturity, sin- 

 gle cells, each with a rather thick wall, 

 which is often studded with bosses, or 

 points, or is variously ridged. In anemoph- 

 ilous plants (see Pollination) the pollen is 

 dry and powdery ; in entomophilous plants 

 it is usually moist and coherent. In milk- 

 weeds and orchids the whole of the pollen 

 from each sporangium is held together in 

 a mass by interwoven threads (Figs. 149, 

 513). By the time the sporangia discharge 

 the pollen, each spore has begun a develop- 

 ment which it completes on the stigma to 

 which it is transferred. See Fertilization. 



833. Pistillate 

 flower of 

 Willow. 

 Showing one 

 compound 

 pistil; s, stig- 

 ma; si, style; 

 o, ovary. 



Carpels. The carpels are the sporangial 

 leaves which occupy the center of the 

 flower. The number of carpels is very 

 variable. Usually they are fewer than the 

 floral leaves. In most flowers the carpels 

 are united one to another to form a structure known as 

 a compound pistil (Figs. 825, 833, 835, 836). When the 

 carpels are separate, each develops as a simple pistil. 

 Of these there may be one or many (Figs. 834, 837). 



The pistil, if simple, first appears as a ring-like 

 ridge about the center of the torus. If compound, knob- 

 like rudiments of the component carpels first appear, 

 but the growth early involves the torus between, giving 

 rise to an elevated circular ridge. This carpellary ring 

 gradually grows upward, partially or completely inclos- 

 ing one or more chambers, in 

 which the ovules arise. At a time 

 when the ovules (which ripen into 

 seeds) were supposed to be com- 

 parable to the eggs of animals, 

 the larger chambered part of the 

 pistil in which they are formed 

 was called the ovary, a name 

 which it still retains in descrip- 

 tive botany. The pistil is often 

 prolonged above the ovary. This 

 part is the style. 



An ovule is a fleshy sporangium, 

 jacketed by one or two (rarely 

 three) outgrowths from the base, 

 the integuments, which almost 

 inclose the sporangium proper 

 (nucellus). Within the sporan- 

 gium of the ovule, several (1 to 40) spores begin to de- 

 velop. Of these, however, rarely more than one reaches 

 maturity. This spore is never set free as the pollen 

 spores are. It therefore acquires no thick wall, and in a 



Head of simple pis- 

 tils in Hepatica. 



section of the sporangium appears as a cavity with- 

 in the delicate tissue which surrounds it. It later 

 becomes the so-called embryo sac, 

 within which occurs the process of 

 fertilization (which see). 



The ovules commonly arise upon 

 certain lines or ridges which project 

 into the pistil chamber, more rarely 

 upon the whole interior surface of the 

 ovary. These lines or ridges are called 

 placentae. See Figs. 825, 835, 837. 



In a simple pistil there is usually but 

 one pi ae enta (Fig. 837). In a compound 



835. Section across 



ll ? e con 2P. und 

 Pistil of Tuhp. 



Showing central p i s til the union of the carpels may be 



?hree C -chamber n ed such as *> P roduce a limbered 

 ovary. ovary, or the ovary may contain as 



many chambers as there are carpels 

 (Fig. 835). In the former case the placentae will project 

 inward from the wall of the ovary ; in the latter they 

 will be aggregated at the center, from which 

 they may project outward into the chambers of 

 the ovary. When the ovules are numerous, the 

 placenta are often enlarged to form an ade- 

 quate surface for their attachment, as in the 

 potato and tomato (see also Fig. 837). 



In a considerable number of plants the 

 ovules arise upon the torus itself, a ring of 

 which grows upward, cup-like. From the 

 edge of this cup arise the floral and sporangial 

 leaves, the ovules developing on its sides or 

 base. The carpels then form a mere roof over 

 the ovule chamber. 



The style is sometimes slender and very 

 long (up to several inches; see Fig. 836); 

 sometimes short and thick (Fig. 833). Its 

 length and form are adapted to the means by 

 which the pollination of the pistil is secured. 

 In some cases the style is practically wanting. 

 Its interior is occupied by a tissue whose 

 cells are pushed aside and partly digested 

 by the growing pollen tube (see fertili- 

 zation). It is not infrequently traversed by 

 a canal, a prolongation of the ovule chamber. 

 Some portion of the style, or when that is 

 wanting a portion of the outer surface of the 

 ovary itself, is adapted to the reception of 

 the pollen spores. This receptive surface, g36. Corn- 

 whatever its form or location, is called the pound pis- 

 stigma (Figs. 833,836). In many cases the til of catnip. 

 upper part of the style is enlarged in to. a Showing 

 knob or club-shaped or lobed portion, the 4 -parted 

 area of the receptive surface being thus in- ovary, long 

 creased. In other cases the style is elongated, style,2stig- 

 and the receptive surface is a long line upon 

 one or more sides of the elongated style. In other cases 

 the style is mucn branched, as in the grasses, and these 

 branches constitute the stigma. At the time the pollen 

 is being discharged, the stigmatic surfaces are often 

 covered by a sticky secretion. All of these devices are 

 adaptations to insure the lodgment, adhesion and nutri- 

 tion of the pollen spores (see Pollination). 



837. Section across simple pistil of May Apple. 

 Showing single placenta and ovules . 



The stimulus resulting from fertilization often accel- 

 erates the growth of the pistil or causes it to resume 

 growth if it had ceased. The various changes in size, 

 texture, color, etc., result in the production of fruit. 

 CHARLES REID BARNES. 



