Angiosperms Figure 38.4 Development of angiosperm gametophytes Life cycle, fruits, seeds 1
Angiosperm seeds consist of diploid and triploid tissues Embryo: Diploid (from fertilized egg) Food Supply: Triploid Endosperm (from polar nuclei and second sperm) Seed Coat: Diploid (from ovule wall) Why double fertilization? According to one hypothesis, double fertilization synchronizes development of food supply (endosperm) with development of the embryo: if no fertilization of the egg occurs, no resources are wasted on building the food supply 2
Remember: Fruit = mature ovary. Pictured below is one kind of dry fruit a legume. Layers of the pericarp Exocarp: outermost skin Mesocarp: in the middle Endocarp: innermost layer, closest to the seed Note: As the pericarp develops, it often differentiates into distinct layers, the exocarp, mesocarp, and endocarp (oft. most conspicuous in fleshy fruits) 2 main kinds of fruit: dry vs. fleshy Figure 38.9 Classification of Fleshy Fruits One carpel / one flower Many carpels / one flower Many flowers Figure 38.9 Classification of Fleshy Fruits Berry: All parts of pericarp are fleshy/pulpy except for exocarp (skin) This grape is a berry. A pepo (pumpkin) is a berry with a thick, inseparable rind. There are 3 main kinds of simple fruits A hesperidium (citrus, orange) is a berry with a leathery, separable rind. 3
Drupe (stone fruit): Usually one-seeded with a stony endocarp, fleshy mesocarp, and a thin and skin-like exocarp. Pome: develops from flower with inferior ovary and compound pistil. Receptacle / floral tube becomes major fleshy part of the fruit. From receptacle Outer limit of pericarp Dry fruits: dehiscent vs. indehiscent. Dehiscent fruits split open at maturity Indehiscent fruits do not split open at maturity Fruit and Seed dispersal Fruits protect seeds during development and sometimes aid in their dispersal Fleshy fruits or seeds are adapted to animal dispersal Dry fruits can be adapted to air or water dispersal, animal dispersal, or to release the seeds at maturity Seeds themselves often have their own dispersaladapted morphology, and adaptations for survival and germination Fruit adaptations that enhance seed dispersal: Red berries are animal dispersed, while dandelion fruits are wind-dispersed (right). Figure 30.x2 A bird eating berries containing seeds that will be dispersed later with the animal's feces. 4
Some fruits, such as these burrs, hitch a ride on the fur of animals The seeds of many plants have elaiosomes fleshy attachments which attract ants. Ants carry the seeds back to their nests, eat the elaiosome, and often discard the seed. (One example is our native wild ginger, Asarum caudatum) Don t forget: many plants also reproduce asexually. Two examples: the maternity plant (Kalanchoe, left), aspen (Populus) groves (right) Seed adaptations for survival and germination Many seeds exhibit dormancy, a temporary condition of low metabolism and no growth or development. Some seeds can survive like this for decades or more. What are the potential benefits of dormancy? Dormancy in some seeds is simply broken by favorable environmental conditions, but others only germinate after specific cues What would you expect the cues to be for seeds living in deserts, fire-prone habitats (such as California chaparral), or at high latitudes? How about for seeds borne in berries eaten by mammals? Figure 38. 8 Review: Three types of seed structure Figure 38. 8 Review: Three types of seed structure 5
Figure 38. 8 Review: Three types of seed structure The four steps of seed germination: 1. imbibition of water, 2. enzyme digestion of stored food, 3. embryo begins growth and radicle is pushed through the seed coat, and 4. shoot tip grows toward soil surface. Germination of a barley seed is shown below. Figure 38.10 Two ways that young shoots break through the soil surface. Figure 38.7 The development of a dicot plant embryo 6