CHPT 28
The two types of spore-producing structures in heterosporous species are often found on the same individual
1. Microsporangia (singular: microsporangium) are spore-producing structures that produce microspores. Microspores develop into male gametophytes, which produce the small gametes called sperm by mitosis. 2. Megasporangia (singular: megasporangium) are sporeproducing structures that produce megaspores. Megaspores develop into female gametophytes, which produce the large gametes called eggs by mitosis.
Once the first green plants made the transition to survive out of water, though, growth on land offered a bonanza of resources.
• Light On land, the amount of light reaching plant leaves is plentiful. In contrast, the water in ponds, lakes, and oceans absorbs and reflects light. As a result, the amount of light available to drive photosynthesis is drastically reduced even a meter or two below the water's surface. • Carbon dioxide CO2—the most important molecule required by photosynthetic organisms—is more readily available to plants growing in air rather than water. In water, CO2 diffuses into leaves more slowly than on land Natural selection favored early land plants with three main adaptations that solved the drying problem by (1) preventing water loss, which kept cells from drying out and dying; (2) providing protection from harmful ultraviolet (UV) radiation; and (3) moving water from tissues with direct access to water to tissues without direct access. Let's examine each adaptation in turn.
The freshwater green algae have long been hypothesized to be closely related to land plants, because key traits are similar in the two groups.
• Their chloroplasts contain the photosynthetic pigments chlorophyll a and b and the accessory pigment β-carotene. • They have similar arrangements of the internal, membranebound sacs called thylakoids (see Chapter 10). • Their cell walls, sperm, and peroxisomes are similar in structure and composition. (Recall that peroxisomes are organelles in which specialized oxidation reactions take place; see Chapter 7.) The earliest land plants even have motile sperm— a reflection of their algal ancestry . • Their chloroplasts synthesize starch as a storage product.
