So far the impression has been given that the only diagnostic feature of the angiosperms is the possession of a carpel. In fact there is quite a list of other characters which collectively designate a plant as an angiosperm. A few of these characters may be seen in some plants which expert botanists consider not to be angiosperms; as with all systems of classification there are always species which represent borderline cases which cannot be placed unequivocally in a group.
However, leaving these aside, the following features appear in most angiosperms:
a) The secondary wood contains many conducting tubes which are known as vessels. These are in the form of long continuous pipes, whereas in the gymnosperms their conducting tubes have cross-walls at intervals. Although these cross-walls have minute pores (pits) in them they do not conduct water in an upward direction anything like so quickly as do the vessels in angiosperms. The latter may therefore keep their stomata open for photosynthesis under conditions where the slower passage of water up the trunks of gymnosperms forces them to close theirs to stop their leaves and twigs drying out altogether. A further advantage of vessels in angiosperms is that it enables them to develop leaves which have a larger surface area and in general are much more delicate than those of gymnosperms. In fact many gymnosperms, especially the conifers, have all the appearance of plants living in areas where there is a shortage of water. However, in most of the habitats in the higher latitudes of the northern hemisphere the annual rainfall is more than adequate for tree growth. Angiospermic trees such as oak, beech and birch which may grow in the same habitats show no sign of water shortage in their environments because their vessels conduct water up to their leaves sufficiently rapidly.
b) A sieve tube in angiosperm phloem is always formed by a process of cell division which produces in addition to the sieve tube a long, narrow cell with a very small cross-section area. This cell is known as a companion cell. It is possible that the separation of the functions between the sieve tube and the companion cell allows the more rapid passage of food-laden solutions through the sieve tube. The latter becomes little more than a mechanism for transport since its nucleus disintegrates and with the loss of the nucleus go many of the characteristics of a normal cell. As a complete contrast the companion cell has every appearance of being one of the most active cells in the whole plant. A very efficient phloem probably confers an advantage to the angiosperms which is denied to the gymnosperms.
c) The angiospermic ovule has two integuments (outer layers).
d) There is a highly characteristic fertilization process. When the pollen tube penetrates the ovule at the micropyle, two nuclei are released. One carries out the essential process of fertilization by fusing with the female nucleus while the other fuses with a second nucleus in the ovule. The nucleus so produced by the second fusion then sets in motion a process whereby a large number of food-containing cells are developed. These provide food for the new embryo.
e) Angiosperms usually have flowers with sepals and petals. Their stamens have a very simple basic structure. Most consist of a filament bearing a bilobed anther with four cavities (loculi).
f) Most angiosperms, excepting those specially adapted to living in difficult habitats, have thin, rather delicate leaves. The pattern of veins in these leaves is usually an extraordinarily complicated network with no part of a leaf a significant distance from the transport system.
Another feature of the lives of flowering plants is the existence of an extraordinary variety of pollination mechanisms involving animals of all kinds. This is not to say that mechanisms involving insects are unknown to the gymnosperms. Indeed, there is evidence that some millions of years earlier than our present point in the history of plants, some gymnosperms were pollinated by insects. There may well not have been any well-developed mechanism, but it probably came about by insects roaming around the surfaces of plants and encountering the pollination drops at the openings of ovules. The production of these droplets of moisture greatly increased the chances of wind-blown pollen grains remaining in place at the openings of ovules and subsequently germinating. Once insects had found these sugary drops containing protein-rich pollen grains a haphazard mechanism could arise whereby the insects, carrying on their bodies wind-blown pollen grains, would visit many ovules on the plant leaving further pollen grains in the process.
From these humble beginnings have arisen a large number of pollination mechanisms in which flowering plants have evolved complicated flower shapes often specially adapted for particular types of animals. At the present day, flowers may be pollinated by bees, flies, wasps, beetles, moths, butterflies, humming-birds, and bats. Even this extensive list omits a number of rarer examples that are known to exist. In order to attract the animals the flowering plants developed scents and also nectar. Pollen in quantities additional to those required to effect an adequate fertilization rate is produced as a further incentive to the animals who use it extensively as food. For human beings, the production of honey by bees is a most welcome consequence of the pollination mechanisms of flowering plants.
Some, like most non-flowering plants, have very successful wind-pollination mechanisms but they do not usually produce brightly coloured, attractive flowers and hence are not picked by human beings. The growths that appear on grass plants are probably not regarded by most people as flowers at all. The larger ones may be used by flower-arranging enthusiasts but that is usually the limit of human interest in the outward appearance of grass flowers.