412 RELATION BETWEEN' POSITION AND FORM OF GREEN LEAVES. 



The leaves of many plants with elongated, erect stems, though at a moderate 

 distance from one another, are often arranged in a kind of rosette, and this is 

 eflected by the stalks of the lower leaves becoming considerably longer than those 

 of the leaves situated near the apex. This condition is especially seen in tlie 

 marsh-plants, whose flat leaves lie on the surface of the water, viz. in Villarsia, 

 Hydrocliaris, Polygonum, ainpJiihium, many species of the genus Callitriche, and 

 many water-inhabiting Raiiunculaceis. Among teiTestial plants this grouping of 

 the leaves is displayed particularly by many Amarantacese. In the erect shoots of 

 Amarantits Blitum, illustrated in fig. 104^, the stalks of the lower leaves of a row 

 are six, seven, or eight times as long as those of the upper leaves. In this way the 

 whole of the green foliage of the plant can be spread out almost at the same level 

 without any one overshadowing another. 



In plants with elongated stems, the mutual encroachment of the numerous 

 leaves situated one above anotlier is also prevented by a further arrangement. We 

 mean the development of the leaves in the form of gi-een scales adpressed to the 

 stem, as observed in so many conifers, e.g. in the twigs of a Thuja, as represented 

 in fig. 103 *. It is true that only the under surface of the small leaflets can meet 

 the sun's rays, but the efiect is the same as if only the upper side had been 

 illumined, as, for example, in those leaves projecting from the erect stems at a right 

 angle, or inclined with their apex towards the soil. Since the small gi-een leaflets 

 clothing the stem are arranged side by side, like the tiles on a roof, and the greater 

 portion of the under surfaces remains uncovered by the adjoining leaves, no 

 mutual withdrawal of light can be said to occur, in spite of the crowded 

 arrangement. 



The arrangements of green leaves as just described relate exclusivelj^ to instances 

 in which the blade of the leaf is neither lobed nor comjjound, but entire. A leaf 

 can deprive another, originating a little below it fi-om the erect stem, having the 

 same shape and size, and the same inclination, either entirely or almost entirely of 

 the sun's rays, only when entire. A leaf whose green lamina is sinuous, lobed, 

 divided, or incised, will always allow abundant sunlight to pass between the lobes 

 and segments on to the leaves below ; and the deeper, wider, and more numerous the 

 incisions producing the separation into lobes and segments, the more will be the 

 light passing through. Of course strips of shadow will be formed, but they move 

 their position during the day, remaining in one spot onlj' for a short time; and it 

 would appear that such a rapidly passing shadow has anjHliing but an injurious 

 eflFect on the green tissue. From this it follows that in plants with divided foliage, 

 the adjustment described previously for the case of entire leaves is superfluous. 

 As a matter of fact, in plants whose foliage-leaves have a much-divided blade, the 

 fully-grown upper and lower leaves are of equal length; they all project from the 

 erect stem at the same angle, and the stem is, generally speaking, never clothed 

 with lobed or pinnate leaves closely covering it like scales. In the Fennel and Dill, 

 in Chamomile, Larkspur, and species of the genus Adonis, the lower and upper 

 leaves of the stem are so alike that it is hardly possible to say whether an isolated 



