so 



kNOWLEDGE. 



[April 1, 1901. 



this. The leaves usually emerge singly. If we join by 

 a line the point of emergence of a leaf with that of the 

 next leaf above it on a stem, and that again with the 

 next, a spiral will be the result, along which at equal 

 intervals we reach the noih^. or points where leaves are 

 borne. And the distance between these nodes will be 

 always found to bear some definite relation to the total 

 length of the spiral line in making one complete revolu- 

 tion round the stem. If the distance from node to node 

 is one-half of this whole distance, it signifies that the 

 leaves are borne alternately on opposite sides of the stem, 

 each leaf being vertically below the second one higher 

 up the stem — a very common arrangement. Or the 

 leaves may be borne three to each spiral revolution, so 

 that the position of each loaf shifts one-third way round 

 the stem as compared with the preceding leaf. If we 

 look along such a stem, the leaves will appear to be borne 

 in three vertical rows, with an equal angle between each. 

 Examining some other jjlant. we may find that we have 

 to go as far as the fifth leaf before we find one vertically 

 above the one from which we started, and if we measure 

 the horizontal distance from any leaf to the next above 

 or below it, it will be found to equal two-fifths of tbe 

 total circumference, so that we have to go five times 

 two-fifths way round the stem, or two complete revolu- 

 tions, before completing the cycle. This is called a two- 

 fifths phj'llotaxis. In many other cases, the arrange- 

 ment is immensely more complicated, and need not be 

 entered on here. What is important for lis to note at 

 present is that by means of this orderly mathematical 

 arrangement, the leaves are so distributed that each 

 fulfils its functions to the best advantage. 



The shape of leaves offers an almost inexhaustible 

 field for observation and scientific speculation. Mr. 

 Rviskin has said: — "The leaves of the herbage at our 

 feet take all kinds of strange shapes, as if to invite us 

 to examine them. Star-shaped, heart-shaped, spear- 

 shaped, arrow-shaped, fretted, fringed, cleft, furrowed, 

 serrated, sinuated, in whorls, in tufts, in spires, in 

 wreaths, endlessly expressive, deceptive, fantastic, never 

 the same from footstalk to blossom, they seem per- 

 petually to tempt our watchfulness and take delight in 

 outstripping our wonder." The size of leaves will 

 naturally vary inversely as their number. A plant of 

 a certain size — say a tree — will require a certain total 

 area of leaf for the manufacture of the recjuisite amount 

 of plant-food. If wc cut the branch of a Horse Chest- 

 nut and of a Beech where each had exactly a diameter of 

 one inch, or two, or six inches, and counted and measured 

 the leaves on each, while the number of Beech leaves 

 would immensely exceed the number of Chestnut leaves 

 the total leaf-area would be about the same in each 

 case. This area of green leaf, then, must be spread oiit 

 to the best advantage. In this connection, a beautiful 

 relation between the shape of loaves and their arrange- 

 ment on the stem may frequently be remarked. Lay a 

 twig of Beech on a sheet of white paper, and note how 

 small are the interstices between the leaves through 

 which the paper may be seen. The shape of the leaves, 

 and the intervals at which they are borne, are so related 

 that an almost continuous expanse of green is offered 

 to the sunlight. A more remarkable case may be seen 

 in the Lime, whose leaves are quite inequilateral, being 

 contracted on one side at the base and expanded at the 

 other, in order the more exactly to fill the space which 

 is available. The Elm likewise furnishes a beautiful 

 example of closc-titting leaves. In most trees in which, like 

 the Beech, Hazel, and Elm, the leaves lie in close-ranked 



rows in the same plane as the twig which supports them, 

 we find more or less oval leaves, their breadth varying 

 with the space between the leaves, i.e., the length of the 

 internode. In trees such as the Horse Chestnut or Syca^ 

 more, on the other hand, the leaves grow in opposite 

 pairs, and are typically arranged on upright twigs, the 

 leaf-stems projecting at a wide angle from the twig, with 

 the surface of the leaf horizontal. In this case space 

 is not so curtailed ; the leaf is lai-ger, and more or less 

 circular in outline ; and the great increase of length in 

 the internodes, as compared with the trees lately con- 

 sidered, prevents a too great overshadowing of the lower 

 leaves by those higher up the shoot. 



In plants which have a very short axis — which have 

 in popular language " no stem " — a difficulty arises as to 

 how all the leaves shall receive a due amount of light, 

 since all arise from the same point. This is met in 

 several ways. The leaves are often placed at different 

 angles, the outer leaves, which are the lowest and oldest, 

 spreading horizontally near the ground, the newest 

 rising almost vertically in the centre, the intermediate 

 being disposed at various angles between these extremes. 

 Another solution of the difficulty is effected by a con- 

 tinued growth of the leaf-stalks, each leaf steadily push- 

 ing itself outv\'ard so that the whole form a slowly 

 expanding circle, in which each leaf-blade successively 

 occupies a position commencing at the centre, ending 

 at the circumference. Such leaf-blades, it is almost need- 

 less to say, are widest at the extremity, since that is the 

 portion which receives most light ; often the blade is 

 roundish, and placed at the end of a bare leaf-stalk, 

 which pushes it further and further from the centre, 

 as other leaves arise. Such arrangements are well seen 

 in many of our biennial plants. During their first 

 season they form a close leaf-rosette of this kind, which 

 manufactures during the summer and winter a supply 

 of plant food to bo stored for the building up of the 



Winter Ir.if- rosette of the Sea Stork's-bill. 



Tt-om a p/((itog/ap7i 7.1/ Mr. H. J. Sev.mot'K. 



tall flowering stem of the succeeding year. The Stork's- 

 bills {'tee figures). Crane's-bills,- Teazel, and other plants 

 will occur to the reader as examples. 



In the case of a few British plants, the normal position 

 of the blade of the leaf is not horizontal, but vertical. 

 The Black Poplar and its relation the Aspen furnish 

 well-known instances. If we examine the stalk of an 



