The Physiological Anatomy of Spartina Townsendii
Total Page：16
File Type：pdf, Size：1020Kb
The Physiological Anatomy of Spartina Townsendii. VY GEO. K. SUTHERLAND AND A. EASTWOOD. With seven Figures in the Text. INTRODUCTION. OPARTINA is a small genus of very characteristic grasses, mainly ^ natives of the Atlantic seaboard of America, where they are to be found abundantly in salt marsh and estuary. Spartina cynosnroides, the freshwater Cord Grass, penetrates inland to the Missouri River, and in the Western States it forms a large part of the grass of sloughs and wet marshes. In Europe only four representatives occur, if we regard S. Townsendii and 6". Neyrauti as one species. 6". juncea is restricted to the western portion of the Mediterranean, whither it was introduced probably by shipping. Of the other three the oldest known is 5. stricta, which Stapf regards as undoubtedly indi- genous. It has the widest distribution of the European forms, but, not- withstanding its long establishment, it is becoming scarce on the south coast of Britain owing to the rapid spread of a later species. 5. alterniflora was recorded first from the neighbourhood of Bayonne at the beginning of last century, and later it was discovered at the head of Southampton Water, down which it spread until its progress was checked by the remaining species, S. Townsendii, about whose origin and first appearance considerable uncertainty exists. In his Flora of Hampshire, published in 1883, Townsend gives the first record of this species as 1878, when it was collected in the neighbour- hood of Hythe, Southampton Water, by the brothers Groves, who described it shortly afterwards as a distinct species. But there is no doubt that it existed earlier, although overlooked. A specimen in the Warner Herba- rium at University College, Southampton, collected near Hythe in 1870 and labelled S. stricta, is undoubtedly S. Townsendii. This carries it definitely [Annals of BoUuy, VoL XXX. No. CXVIU. April, 191O.J Downloaded from https://academic.oup.com/aob/article-abstract/os-30/2/333/2112594 by University of California, Santa Barbara user on 16 March 2018 334 Sutherland and Eastwood.— The Physiological back to that date, but there is reason to believe that it occurred even before that time. The older accounts of the Spartan grasses vary much, and in Sowerby's 'Grasses' (1861) the opinion is expressed that the plants of S. altertii/jora, collected near Southampton, were so like 5. stricta that they could not be regarded in any other light than as intermediate varieties. But 5. stricta is a fairly constant species showing remarkably few varia- tions. Therefore the probability is that the doubtful specimens were in reality plants of S. Townsendii, which in many of its characters is inter- mediate between 5. stricta and 5. alterniflora. Unless the plants were examined carefully in situ, and at the flowering season, the appearance of this new plant might be overlooked for several years, mixed as it was with a very similar species, and growing in places not readily accessible. This view would help us to understand better its present extent and profusion. Townsend's Spartan grass has been characterized by its phenomenal success on the shelving mud banks along the entire western shore of Southampton Water, whence it has spread with amazing rapidity over the available and suitable mud flats between Selsey Bill and St. Alban's Head, which form the natural boundaries of the sunken valley of the old Frome or Solent River. Here the numerous creeks and estuaries, harbours, and salt marshes from Poole to Chichester are well protected, while tertiary forma- tions have supplied abundant mud, Again, the various stages of the pro- gress eastward and westward have been short and favoured by eddying currents, which have helped largely in fruit dispersal. On both sides of these limits, where chalk ridges reach the sea, there are extensive stretches of shingle beach and cliff, broken by few suitable openings, with the result that for the time being the natural spread seems checked. The value of this grass in fixing arid in reclaiming shifting and unsightly mud banks has been recognized, and already attempts have been made to utilize it. Plants placed in the Medway have made considerable progress ; others have also become acclimatized at both Blakeney Point and Wells Marsh in Norfolk. More recently the experiment has been extended northwards to the mud flats of the Forth and Don mouth. At the present time it is the dominant species in the south of England. 6\ stricta survives in a few quiet backwaters, while S. alterniflora is dis- appearing fast before its more vigorous competitor, whose adaptation and success may be gauged by a glance across the Spartan beds from Cadland to Calshot,or from Lymington Harbour to Hurst Castle Bay. It is inevitable that such an extensive and thick vegetation should affect the deposition of silt near river mouths, and hence tend towards a quicker levelling up. At present there exist no reliable data with regard to the rate, but it is hoped that a series of accurate survey measurements may be made at different stations along Southampton Water. Downloaded from https://academic.oup.com/aob/article-abstract/os-30/2/333/2112594 by University of California, Santa Barbara user on 16 March 2018 Anatomy of Spar Una Townsendii. 335 Apart from its usefulness, the grass presents interesting ecological features, and the present study was undertaken in the hope that it would throw some light on its adaptation to a life of periodic submersion, as wel as form an introduction to 'a contemplated series of experiments on its physiology. The distribution and spread of the plant has been treated fully by Stapf, who is still carrying out his observations on its ecology. EXTERNAL MORPHOLOGY. Rhizome. Spartan grass owes its power of rapid extension mainly to its characteristic rhizomes, which vary in length from a few inches to over a foot, rarely exceeding one-quarter inch in diameter. Their length is dependent for the most part on the type of soil and the available space. They travel horizontally through the mud at a depth of from %\ to 4 inches, but frequently in the young stages they show positive geotropism and pene- trate downwards for a short distance. Near their point of origin they are firm, with short hollow internodes. The greater portion of their length, however, is soft and flexible, the whole structure being adapted for pene- trating a soft substratum. Short scale leaves are found in the bud stage, but the mature rhizome is invested merely by colourless sheaths devoid of blade and ligule. After proceeding for a short distance, rooting more or less freely at the nodes, the tip turns upwards and gives rise to aerial branches. New rhizomes are produced sympodially, and in the early stages there is nothing to distinguish them from the aerial shoots, except that the latter remain for a longer time within the sheath and are given an upward tendency. The creeping axes are shorter in a stony substratum, and hence it may also be that the question of the available space plays a part in determining the tendency and consequently the ultimate formation of culm and rhizome. Culm. The erect cylindrical aerial axis, covered almost entirely by numerous investing leaf-sheaths which enhance its rigidity, reaches a height of from a to 4 feet. The first shoot arising from the upturned rhizome tip is usually the dominant one. Secondary culms spring distichously from its basal portion. Of these the lower and outer generally grow more quickly, checking the development of the higher ones. Only at the margin of the clump or belt is it possible for plants to develop all or most of their aerial shoots. Leaf. The leaves are also distichous. The first three or four are practically scales. As soon, however, as the axis rises above the ground short green-tipped blades appear, increasing in size as the stem is ascended, until they reach a length of twelve to eighteen inches. The lowest blades are thrown off at an early stage, separating at the articulation. This helps to distinguish this species from S. alUrniflora, in which the lower blades are retained longer and wither gradually. A short soft-celled ligule, tipped Downloaded from https://academic.oup.com/aob/article-abstract/os-30/2/333/2112594 by University of California, Santa Barbara user on 16 March 2018 336 Sutherland and Eastwood.— The Physiological with hairs, encircles the stem and prevents water or mud from lodging between it and the sheath. The long smooth pale-green sheath clasps the axis firmly, completely surrounding it for the greater part of its length and passing into the blade through the pulvinar articulation. The lower blades form angles of from 450 to 60° with the stem, while the upper ones are more erect. The angle increases with submersion. During the fall of the tide the leaves swing up and down in the surface film until they are suddenly released and spring into position. The adaxial surface of the blade is increased enormously by from forty to fifty ridges and furrows running from just above the pulvinus to the apex. These provide an increased assimilatory surface in addition to pro- tection for the stomata. The papillae and waxy coating on this surface give it a velvety and glaucous appearance. Root. The roots are adventitious and divided into two distinct sets. One type is long, relatively thick, smooth, and practically devoid of branches. This fixing form grows normally to a depth of six to twelve inches, serving to anchor the plant firmly in the soft mud. While its root- cap is large, root-hairs rarely, if ever, occur.