MODIFICATIONS OF FORM 205 



the complete circle. It is found that in many plants, and even in whole 

 families, certain angles are constant in the mature shoot. This gives a com- 

 parative, or even a systematic value to their observation. For example, in the 

 Grasses, and in Iris, the angle of divergence is £, that is, the leaves are alter- 

 nately on opposite sides of the stem, the third being above the first : they thus 

 constitute two longitudinal rows. In the Sedges, Veratrum, and other Mono- 

 cotyledons the angle is \, the fourth leaf being above the first, and their 

 arrangement being in three longitudinal rows. In the Rosaceae and many 

 other Dicotyledons a common angle of divergence is jths, and the leaves are 

 arranged in five rows. Consequently the sixth leaf will be directly above the 

 first, while the imaginary spiral threading their bases together will have passed 

 twice round the axis before reaching it. Other more complex arrangements 

 are expressed by divergences of f (Rhododendron) (Fig. 137), and A [Dracaena], 

 etc. These higher divergences go along with shorter internodes, and compact 

 grouping of the leaves ; while their overlapping is avoided by the spiral 

 arrangement. 



Various theories have been propounded to account for these facts. The 

 old spiral theory assumed an inherent tendency to spiral organisation in plants, 

 and, deductively, attempts were made to read spiral construction into all 

 shoots. Subsequently a theory of contact-pressures was suggested, according 

 to which the spirals resulted from mechanical arrangement of the leaf-primordia 

 upon the axis, comparable to those of marbles in a flat frame. But though 

 such pressures may in certain cases have an effect upon the arrangement of 

 the parts as they mature, they do not explain the initial steps. For when the 

 primordia first appear they are not in contact with one another. In point of 

 fact the exact position of the primordia of leaves upon the axis, and their initial 

 arrangement relative to one another, can at present only be referred to inner 

 causes as yet unknown ; such as localisation of hormone within the growing point 

 (see Chapter XXXVI.). 



Some plants develop with bilateral symmetry, having anterior and posterior, 

 or right and left sides, which are alike. The flattened shoots of the Prickly 

 Pear, or of Phyllocactus, are examples, also certain Mosses (Fissidens). But 

 the headquarters of this type of symmetry, which is uncommon in Flowering 

 Plants, is in the Marine Algae, and a good example is seen in the common 

 Bladder Wrack (Fucus, Fig. 280, p. 379). 



DORSIVENTRALITY. 



While radial symmetry is the rule in upright shoots, those in which 

 the axis is oblique or horizontal usually diverge from the radial in 

 more or less degree. They show an obvious relation to the direction 

 of gravity, light, etc., developing differently on the sides directed 

 upwards and downwards. Such developments are styled dorsiventral. 

 Sometimes the effect appears only in the later development of the 

 axes or appendages ; but in more pronounced cases the initial arrange- 

 ment of the leaves is itself dorsiventral. 



Examples of the former are seen in the lateral branches of many 

 trees, and conspicuously in many Conifers, such as the Spruce. 



