UNE 20, 



I918] 



NATURE 



31 



THE STORY OF A OLWSSA 



GRASSES form one of the largoi and nuisi wide- 

 spread families, adapted to very dilTerent condi- 

 tions of soil and climate, but with a remarkably 

 uniform plan of structure. \\'herever conditions' allow 

 of plant-life on land, there, almost without exception, 

 the family is represented. In number of species the 

 glass family falls short of otiier great famihes of 

 flowering plants, Compositai, Leguminosae, or Orchids, 

 but in the aggregation of many individuals of one and 

 the same or a few species, either growing alone or 

 densely scattered through a mixed herbage covering 

 large areas, it forms a pre-eminent tj-pe of the earth's 

 vegetation — as, for instance, in the grass-carpets form- 

 ing the meadows or pastures of temperate or cold 

 climates, or the coarser growth prevalent over vast 

 areas, as in steppe or prairie vegetation. 



These sociable grasses play an important part in 

 the general scheme of plant-life ; they protect the soil 

 from too great evaporation of water, and .cover up 

 other plants in the resting stage, such as bulbs, tubers, 

 etc., during the cold or dry season. The penetrating 

 effect of the roots and underground stems helps to 

 break up a stiff soil and fit it for other plants. 

 Examples of the great variety of habitat in which 

 grasses thrive are seen in the short turf which covers 

 Kmestone areas, where the soil is too dry and thin 

 to support trees or shrubs ; in the luxuriant growth 

 of meadow-land where it thrives together with a 

 variety of other herbs ; in the reed-grasses which are 

 associated with water; in the coastal mud-flats in 

 Hampshire and Sussex, which are being rapidly re- 

 claimed by the growth of Spartina Townsendi, a 

 vigorous-growing hybrid which has spread over large 

 areas during recent years ; or the sand-dune grasses, 

 which bind and fix the sand dunes and prepare the 

 way for a more varied and permanent type of 

 vegetation. 



The adaptation necessary to accommodate the plant 

 to widely differing conditions of life does not involve 

 changes in general plan of structure; for instance, 

 in hot, dry, or exposed areas, where excessive loss of 

 water by the "plant must be avoided, this is effected 

 bv narrowing the leaf-surface, or roiling it over from 

 margin to margin to protect the upper face on which 

 are the water-transpiring stomata. The structure of 

 the stem, a slender, hollow cylinder, strengthened by 

 a band of supporting- fibres beneath the outer layer, 

 or strips accompanying the water-conducting tissue, 

 gives sufficient strength, with the greatest economy of 

 material, for the purpose required, namely, to carry 

 up into the light and air the leaves, flowers, and fruits 

 for the short period of active life, and to allow of the 

 swaying motion which favours the processes of nutri- 

 tion, of transfer of pollen, and of distribution of the 

 mature fruits. 



The mode of development of the branches at the 



~ of the stem determines the habit; a tufted growth 

 ilts from the upward growth of the buds in the 

 lior of the leaf-sheath, as seen, for instance, in 

 •'tillering" of cereals; while the turf- or sod- 

 nation is due to the penetration of the sheath-base 

 the voung shoot and its lateral development in the 

 -. il. Branching from the upper part of the stem is 



:< in grasses of the temperate zone, but occurs in 

 pical genera, and is characteristic of the bamboos, 

 which the woody stem often attains tree-like 

 portions. 



!\)ints of interest in the structure of thr ^la^^es are 

 mode of growth in length of tlir >ti in 1)\ a zone 



!M:ot!r-ie Uelivered at the Royal InsUtution on M.^y 17 liy 

 I.R.S. 



IsO. 2538, VOL. lOl] 



of growth above the place of insertion of each leaf, 

 the rigidity of the stem at this tender-growing zone 

 being maintained by the stiff, encircling leaf-sheath ; 

 the swollen " node " round the base of each leaf-sheath, 

 which is able by a geotropic growth response to an 

 alteration in its position to raise again to a vertical 

 position a stem which has been laid; and tin shot i 

 prolongation of the leaf-sheath above the line i>i it> 

 union with the leaf-blade to form ilir membranous 



I "ligule" which protects the entrancr to the tube 



I formed by the sheath. 



The grass-flower and the association of flower-; lo 

 form the inflorescence are very charac n ri-ti( . The 

 unit is the spikelet which contains one to several, 

 sometimes many, flowers. The character of the spike- 

 let is determined by the bracts or glumes, the green 

 membranous or papery scale-leaves which enclose the 

 single flower and overlap each other in a double row- 

 when several flowers are present in a spikelet. The 

 bract-leaf is a general method of protecting tin llower- 

 bud ; in the iris, for example, each flowir-bud is 

 enveloped by a pair of bracts — the outer, farther from 

 the main stalk, green and leaf-like, the inner, between 

 the bud and the main stalk, thin and hyaline with a 

 double keel on the back. In the iris the bracts wither 

 as the flower opens, in the grass the bracts remain 

 as the character-giving feature during flowering and 

 fruiting, the flower itself being reduced to those organs 

 which are directly coVicerned in the setting of the fruit. 

 The pollen is distributed by means of air-currents, and 

 the petals are represented merely by a pair of minute 

 fleshy scales (lodicules) at the base of the flower, 

 which, by absorbing water, swell and cause the bracts 

 to separate, and thus allow the thread-like stamens to 

 grow out and expose the delicately hung anthers, from 

 which the light dusty pollen is scattered by the wind; 

 the feathery stigmas protrude later to catch the pollen- 

 grains. In the great majority of grasses there are 

 three stamens, as in the Iris family, and a single 

 ovary bearing a pair of long, feathery stiginas and 

 containing a" single egg. The remarkable variety in 

 the form of the spikelet and the inflorescence is 

 achieved by variety in the form, size, .and number of 

 the glumes which constitute the spikelet and the degree 

 of branching of the inflorescence. The colour of the 

 inflorescence is due to the colour of the pendulous 

 anthers, and disappears when these drop after shedding 

 the pollen. Fertilisation of the ovule succeeds pollina- 

 tion of the stigma, and the ovule becomes the seed, 

 which, e.xcept in a few genera, is permanently en- 

 closed in and inseparable from the fruit. The fruit 

 also generallv remains enclosed in one or more of the 

 glumes, which fall with it and by their light, papery 

 consistency help in its distribution by wind. Fre- 

 quently the outer glume bears a stiff aw-n on the back 

 or tip," which is an effective aid to distribution, as it 

 will cling to the coat or plumage of an animal or 

 bird. In the steppe grasses of the genus Stipa the 

 awn is sometimes very long and feathered, forming an 

 admirable device for distribution by w ind. The stiff 

 awn is frequently spirally coiled in its lower portion 

 and hygroscopic, and its coiling or uncoiling with the 

 varying degree of moisture in the atmosphere is 

 arranged so as to drive the pointed end of the glume, 

 in which the fruit is enclosed, into the ground. 



The seed contains the embryo at the lower part 

 of one side ; the rest consists of a food store of starch 

 and gluten to nourish the embryo on germination. 

 The embrvo has a well-developed stem-bud or plumule 

 and root; the plumule is enveloped by a sheath 

 (coleoptile), which appears above ground in germina- 

 tion as the slender pointed green seed-leaf from which 

 the true leaves successively break. The food store in 



