6o8 



NA TURE 



lApril 29, 1886 



drain, and so come into contact with water in quantity ; 

 others remaining unchanged, and utilising only the water 

 of the soil. Such differences maybe noticed also in the 

 case of hyacinths, grown some by water culture in glasses, 

 others in ordinary earth. The former roots are larger 

 and more succulent than the latter. A difference also 

 may be seen in the development of the root-hairs, though 

 a very definite statement about this can hardly be made. 

 Still, in allied species, and often in individuals of the 

 same species, the hairiness of the roots increases with 

 increasing sunlight, dryness, and airiness of the spot in 

 which the plants are growing. 



The leaves also undergo much structural modification. 

 Many plants have leaves which are totally submerged, 

 and these are able to resist the action of the water, which 

 would soon destroy ordinary leaves, whose constitution 

 fits them to live only in air. Some amphibious plants 

 show this peculiarity well. They grow generally in 

 marshy places, or on the banks of streams, by which they 

 are often submerged. Such a plant, having its land form, 

 possesses leaves which die on being submerged, but later 

 it puts out other leaves which are not injured. In Lycopus 

 eitropceus and in LythruDi Salicaria there is also a histo- 

 logical difference between the stems grown in water and 

 those grown in air. Two layers of cells containing no 

 chlorophyll are developed in the watery specimens deep 

 down in the cortical parenchyma The outer layers of 

 tissue perish, and these new cells then serve to protect 

 the tissues within. In the leaves of water plants also, 

 other peculiarities are noticeable. Generally chlorophyll 

 is developed in the epidermis, a fact which is perhaps 

 connected with the slight amount of evaporation taking 

 place The position of the stomata and their relative 

 number in different cases is also closely connected with 

 their habit of life. This may be well seen in Marsilea, 

 whose leaves, though generally raised above the surface 

 of the water, are sometimes to be found floating on the 

 surface. The aerial leaves have stomata on both upper 

 and under surfaces, but the swimming ones have them 

 only on the upper surface, and have then three times the 

 number that the same surface of the aerial leaf possesses. 

 It is easy to show that the change is the result of the 

 change in the environment, for if a piece of the plant, 

 possessing quite young leaves, be submerged and kept 

 under the surface, the young leaves will develop into 

 swimming-leaves, and not aerial ones. In other water 

 plants with large floating leaves the same disposition of 

 stomata may be seen. Generally on sub-aerial leaves it 

 is the lower surface which shows them in far the larger 

 quantity. The environment of the plants then seems to 

 have a great influence on their distribution, that arrange- 

 ment being followed which is best suited to keep the 

 stomata dry. 



A curious adaptation of structure to environment is 

 seen in the roots of the epiphytic orchids and aroids. 

 These are aerial in habit, hanging freely downwards. 

 Not coming into contact with water in the same way 

 as either aquatic or terrestrial plants, they have no 

 root-hairs. There is a development of tissue met with 

 in them which enables them to absorb and avail them- 

 selves of the moisture in the atmosphere. Instead of the 

 usual single-layered epidermis, they are covered by 

 a many-layered velameu made up of numerous cells 

 fitting closely together, of the description known as 

 tracheides. The usual cuticle or secretion from the outer 

 walls of the epidermis, which is always very little deve- 

 loped in roots, is here altogether absent ; the membranes 

 of the cells are .usually colourless, and the cells them- 

 selves contain air. This layer absorbs water quickly, 

 supplying the plant with moisture. 



The influence of the environment on the forms of 

 leaves, as well as on their structure, can be well seen also 

 in aquatic plants. The swimming-leaves show certain 

 general resemblances, their form being more or less 



rounded, and not as a rule lobed or cut ; they are, too, 

 usually of fair dimensions. In the case of submerged 

 leaves we find differences which are connected with the 

 conditions noticeable in the water. Thus, in a rapid 

 stream they are generally long and very much divided, 

 while in stagnant water this is not the case. Three 

 species of the genus Ranunculus especially exhibit a 

 gradation in this respect. R. divaricattis is a denizen of 

 stagnant water ; R. aquaiilis is found in slowly-flowing 

 streams; /f.^/c/Zawj- in rapid ones. The divisions of the 

 leaf are longest in the last case and shortest in the first, 

 the second being intermediate. A', aquaiilis is an am- 

 phibious form, and shows well how environment decides 

 the character of the adult plant. When growing in a 

 pool it has its leaves in fairly long divisions, the 

 lobes being rounded and the internodes long. If 

 the pool should dry up, it changes gradually, the new 

 leaves being less divided and the divisions becoming 

 flattened, while the nodes are nearer together. The epi- 

 dermis, which in the water form had almost square walls, 

 now becomes serpentine. Growing so, it produces in due 

 time its flowers and seeds, and these latter reproduce the 

 land form. If the pool again becomes filled with water, 

 a reversion speedily takes place, and again the character- 

 istics of the water form are seen. The two are, in fact, 

 easily converted from the one into the other. 



Not only is the correspondence between environment, 

 form, and structure seen in the species of Ranunculus 

 already alluded to, but the whole genus can be divided 

 into two sections, those of terrestrial and those of aquatic 

 habit, so nearly allied to each other in all points of so- 

 called systematic importance that they are now included 

 together under the common name Ranunculus, and yet 

 e,\tremely dissimilar in form and structure of the vegeta- 

 tive parts. The same difference in amount of woody 

 tissue as has been alluded to above' can be seen most 

 strikingly by comparison of sections of the stem of K. 

 7-epcns with those of the stem of R.fluitans. 



Turning now from aquatic plants to those which, 

 though alike terrestrial, are yet situated amid very differ- 

 ent surroundings, the effect of the environment can easily 

 be traced. Take, first, the plants which inhabit regions 

 in which habitually the air contains very little moisture. 

 These may be affected in several different ways— the 

 most conspicuous modification perhaps being the differ- 

 ent forms of succulent plants, such as Mcsembryanthe- 

 mtim. In these the leaves have lost the usual ratio 

 between surface and bulk ; they are now thick and fleshy, 

 their internal parenchyma being very succulent or pulpy ; 

 their outer layers leathery, with comparatively few 

 stomata, and a great reduction of the system of inter- 

 cellular spaces typically found in the leaf Their en- 

 vironment has led to such a structure as will enable them 

 to make the most of the limited supply of moisture, great 

 facilities being seen for storing it, and precautions taken 

 against its escape. Similar adaptations, but affecting the 

 stems and not the leaves, are found, f,^'., in the branching, 

 fleshy Opuntias, while we have also large, thick, fleshy 

 leaves in the aloes and agaves of such regions. The 

 genus Euphorbia has some strange repiesentatives here. 

 There are several hundred species of this genus, inhabit- 

 ing all parts of the world, and all characterised by the 

 peculiar structure of the flower familiar to us in the 

 common spurge of our gardens. The great majority of 

 the species are annual or perennial herbs, with slender 

 unarmed stems bearing great numbers of scattered, sessile, 

 simple leaves. The comparatively few members of the 

 genus which inhabit the regions of little moisture have 

 become so extremely modified in their vegetative parts as 

 to closely resemble cactuses. E. canarieiisis especially 

 has taken on this peculiar habit, developing enormously 

 its stem and branches, the former becoming in sonie 

 cases 20 feet high, and ceasing to produce leaves, while 

 the branches are plentifully supplied with prickles. 



