Lots to explain here, for the complete picture.
The "stem" with flowers is called an inflorescence. All Echinodorus species (the common Amazon sword is most likely E. grisebachii, the "bleherae" variety as often termed) reproduce sexually or vegetatively, depending upon how they are grown. In their habitats, most are amphibious bog plants, spending roughly half the year emersed (growing in very damp soil, leaves fully in the air) and half submersed (the wet or rainy season when the forest floods and the plants are fully underwater). When emersed, the inflorescence will flower (sexual reproduction) and produce seeds; if grown permanently submersed, as in the aquarium, flowers are rarely if ever produced, but daughter plants called adventitious plants will appear from each node, two per node, along the inflorescence.
Nurseries propagate sword plants emersed because it is less expensive and much faster. So when we buy one, it may have flowers. The other thing though is that the air must be very moist, as it would be in the tropics, so the adventitious plants should either be kept submersed at or near the surface, or the tank may be well covered to keep the air very moist and warm, humid in fact. But the light may still be too intense...in their habitat, these plants do not see much direct sunlight because of the forest cover.
Re the CO2. Aquatic plants that grow on the surface, or above it, have what we term the aerial advantage. And this is the ready supply of CO2. It is about four times slower for plants to assimilate CO2 from water than from air. Lower leaves on the parent plant should not be affected by this; once the adventitious plants form leaves and tiny white roots from the nodes, they are no longer deriving any nutrition from the parent plant. You can leave the inflorescence, cut it off, remove some or all of the adventitious plants, whatever. I sometimes leave some plants along the inflorescence as it can be very effective in the aquarium. But one thing I find is that being closer to the surface and the tank light, these plants frequently develop brush/beard algae.
Echinodorus plants are quite heavy feeders, so a root tab next to the roots in the substrate will help a lot. I use Seachem's Flourish Tabs which seem better than the API brand. A comprehensive liquid fertilizer is also beneficial, just don't overdo it, as it can cause brush algae. Especially under a T5 HO tube, which is quite intense light. Some floating plants here would probably help.
Yellowing or dying leaves on the parent plant are normal when the plant is first acquired. It has been growing emersed, and the emersed leaves are different in structure from submersed leaves that will form when the plant is grown fully submersed. You may see quite a change in the leaf shape. This rather depends upon many factors, including the plant itself. This species, E. grisebachii, is now known to contain eight varieties (in addition to the original E. grisebachii) that were formerly considered distinct species, and they have quite differing leaf forms. One may still see plants named E. amazonicus, E. bleherae, E. parviflorius, E. gracilis, and some others, but after detailed phylogenetic analysis, they are all botanically the same species, E. grisebachii, according to the DNA.
Byron.
