190 



PHYIUM TRACHEOPHYTA: 



are derived from a single carpel or groups of carpels 

 and sometimes from other floral parts as well. 



Occurrence: include the vast majority of living 

 plants; about 200,000 living species found throughout 

 the world; mostly land plants and free-living, but 

 there are epiphytes, parasites, animal predators in the 

 form of certain species that utilize mostly insects as 

 food, as well as many freshwater and even a few 

 marine forms. 



May be no more than a group of modified leaves, 

 only certain kinds with sporangia. This interpreta- 

 tion is generally accepted for various reasons. For 

 example, parasitized flowers often form into a group 

 of leaf-like structures. A flower is one of the few or- 

 gan systems found in plants. 



Structure: commonly borne upon an inflated part 

 (receptacle) of a special flower stalk (pedicel); typi- 

 cally composed of four whorls of distinctive parts; 

 from outermost to innermost whorls, composed of a 

 whorl of sepals (collectively called the calyx), of a 

 whorl of petals (collectively called the corolla; calyx 

 and corolla collectively called the perianth; any peri- 

 anth part called a tepal, a useful designation when 

 either the calyx or corolla is absent and one cannot 

 determine whether petals or sepals are present), of a 

 whorl of stamens (collectively called the androecium), 

 and of a whorl of pistils (collectively called the gynoe- 

 cium); pistils and stamens comprise the so-called es- 

 sential organs ofa flower (Figure 11.28). 



Sepals: regularly green and leaf-like, sometimes 

 colored and like petals. They enclose the unopened 

 developing flower (bud) and may or may not remain 

 during the life of the flower. 



Petals: often colored, larger than the sepals, and 

 fragrant. They are borne singly or fused together 

 (petal number then being indicated by apical petal 

 lobes). 



pisti 



anther ^.^ 

 filament->^*°'^®" 



-petal 



sepal 



receptacle 



Figure 1 1 .28 Ports of o flower. 



Stamens: vary greatly in size and organization; 

 each usually consists of two parts, anthers, containing 

 one or more, usually four, male sporangia called pol- 

 len sacs and a sterile, hair-like filament (or stalk) that 

 attaches the anther to the receptacle. 



Pistils: basic unit the carpel. They are either 

 simple and composed ofa single carpel, or compound 

 and composed of two or more fused carpels. Each 

 carpel (pistil unit) of a compound pistil usually can 

 be distinguished either by cross-sectioning the ovary 

 and counting the number of ovary partitions, or by 

 counting the number of style or stigma lobes. 



SEED FORMATION 



Fertilization: pollen is specialized to travel by 

 wind, organisms (mostly insects), and/or water; 

 normally mechanisms prevent self-pollenation by a 

 plant; flower stigma secretes a sticky, sugary fluid 

 which catches and perhaps stimulates pollen germi- 

 nation; upon contacting a stigma, a pollen grain ger- 

 minates, growing between stigma cells, down the style 

 (often a hollow structure), generally enters an open- 

 ing to the embryo sac (female gametophyte); when 

 the pollen tube ruptures, two sperm and often some 

 cytoplasm including a vegetative cell are discharged; 

 then, double fertilization because one sperm fertilizes 

 the egg and the other sperm joins with the two other 

 female gametophyte nuclei to produce a triploid (3n, 

 or three chromosome sets) endosperm nucleus; the di- 

 ploid zygote is the first cell of the next sporophyte 

 generation and the endosperm nucleus is the fore- 

 runner of endosperm (a nutritive seed tissue for the 

 germinating sporophyte) (Figure 1 1.29). 



Apomixis: some angiosperms have a special proc- 

 ess of seed and embryo formation that does not in- 

 volve fertilization of the egg; such seeds often have 

 more than one embryo (sexual seeds rarely do). 



Development: a mature embryo consists of a pri- 

 mary root, a primary shoot with a shoot apex, and 

 one (most Subclass Monocotyledoneae) or two (most 

 Subclass Dicotyledoneae) embryo leaves; endosperm 

 cell divides to produce endosperm tissue. 



Seed Production: embryo and endosperm generally 

 are not covered by the female sporangium wall, or 

 nucellus; however, the tissues surrounding the female 

 sporangium (integument) generally become thick, 

 layered, and form a very "hard seed coat. 



Fruits: seeds are usually covered by the ripened 

 ovary wall and often certain other ripened structures. 



Germination: seeds normally require a resting 



