Some difficulty arises in connection with terminal awns. In some cases there is a true awn—i.e. a distinct bristle or hair—at the apex of the palea, and apparently continuing its substance without interruption; but in most instances close examination shows that this awn arises from between two minute teeth, and is really inserted at the back of the slight depression between them—e.g. Lolium temulentum (Fig. [48]), Brachypodium pinnatum (Fig. [77]), &c.
In another class of cases the awn appears to be really the prolongation of the palea—e.g. Nardus (Fig. [81]), Festuca Myurus (Fig. [80]), &c.—and when it runs out into a distinct bristle we may speak of a terminal awn without staying to discuss whether or no it is really terminal in development.
In Arundo, Cynosurus and some Fescues, where the palea tapers off into a stiff long point, I have not spoken of it as an awn, but have described the palea as tapering into a sharp point (awn-point). It must be admitted that the distinction is somewhat artificial, but it has its advantages in practice.
| Fig. 50. Bromus sterilis. a, “seed," nat. size; b and c, ditto, × about 2. Nobbe. Cf. Figs. 80 and 81. | Fig. 51. Festuca sylvatica. a, "seed," nat. size; b and c, ditto, × 8. Nobbe. Cf. Fig. [78]. |
The rachilla (Fig. [41]), when present, often affords good characters, and in a few cases is relied on for the distinction of “seeds" otherwise much alike—e.g. Lolium perenne and Festuca elatior; and similarly with the presence or absence of hairs (Digraphis, Arundo, &c.) or “web" (Poa) at the base of the “seed." (Figs. [55], [69].)
On germination the primary root of the embryo usually emerges and at once plunges into the soil, but soon ceases to grow, and the secondary roots (and subsequently adventitious rootlets from the lower internodes) soon give the peculiar fibrous character so well known in grass-roots.
The plumule either pushes out from the same end of the caryopsis as the root (e.g. Wheat, Rye) or drives its way between the coats to the opposite end (e.g. Barley) and appears as a pointed cylinder of rolled leaves (the "spear") the outermost of which is sheath only, no lamina being developed. As Darwin showed, the pointed apex of such a plumule is hardened, facilitating the piercing of the soil above, and when the blade attains the open air it performs spiral movements during growth, indicating that similar attempts to rock from side to side have aided the plumule in forcing its way through the soil. It has also been shown that the apices of roots and plumules are sensitive to differences of temperature, of light-intensity, moisture and contact, all of which aid the seedling in establishing its position in the soil and in liberating the "spear." For particulars the student should read Darwin’s Power of Movement in Plants.
With regard to the established seedlings of grasses, many interesting details of structure are to be found in them. I have not sufficient material to draw up a diagnostic arrangement of grass-seedlings, but it is evident that such could be done. It may be useful to illustrate this by the following tabular view of the characters of the larger seedlings of our common cereals, in part adapted from Vesque and Percival; though I find that some variations may occur, especially in the development of the auricles.