516 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



garden Larkspur. The petiole of this plant is hollow, precisely like the 

 stem. When such is the case it is cylindrical and not grooved. 



As an illustration of the way Nature meets a strain, Darwin describes 

 how the petiole of a leaf of Solatium jasminoides increases its wood after 

 having caught hold of some support. It has at first three coherent cords 

 below, and two small ones above ; but subsequently they become united 

 into a cylinder by the intercalation of intermediate ones.* 



There are many plants which have no blades to the petioles. When 

 this is the case the latter become flat, and in some plants have their 

 surfaces in a vertical plane. Such is a peculiarity common in Australian 

 Acacias. They are called phyllodes. The meaning of this arrangement 

 appears to be to guard against injury by too great a radiation of heat ; 

 another use of petioles, often seen in members of the Umbelliferce, is to 

 form a bag-like protection for the large flower-bud within it. 



Akin to this use is the reduction of the petioles to make bud-scales. 

 This is obviously the case in Bibes, Ash, and Horse-chestnut, as transi- 

 tions to perfect leaves with blades can readily be seen on dissecting an 

 expanding bud in spring-time. 



In Monocotyledons, the stems have their cords scattered about the 

 pith or ground- tissue. When a leaf is going to be formed, the cords 

 usually throw out branches which interlace at the node, and so form a 

 plexus, as described in Bryony. From this a number of cords pass out 

 near the circumference and enter a flat leaf with a sheathing base. As 

 the cords run parallel from end to end, monocotyledonous leaves are 

 distinguished as a rule by being parallel or straight-veined. 



In some water-plants, however, this long narrow leaf may ultimately 

 develop a blade, as may be well seen in the Arrow-head (Sagittaria 

 sagittifolia). This would seem to reveal the fact that the parallel- veined 

 leaves of Monocotyledons are really of the nature of phyllodes ; such 

 being originally formed under water, as on the above-named and 

 many other aquatic flowering Monocotyledons. 



If the water be deep, no blades are formed in the Arrow-head ; but if 

 it be shallow, then, first, an elliptical blade appears, followed by a hastate ; 

 then a sagittate blade is the final result.t 



Simple or One-bladed Leaves. — The two extreme forms of blades 

 are linear, i.e. very narrow, and orbicular, or completely round. Grass 

 blades illustrate the former, and Water-lily leaves the latter. 



The varying breadth of blades is found to be largely dependent upon 

 their position and exposure to the sun. When spread out horizontally 

 the greater becomes the surface, as it can receive more light and con- 

 sequently acquires a greater assimilative power. 



If, on the other hand, the leaves of herbaceous plants be greatly 

 crowded, as of Grasses, Sedges, Carnations, Thrift, &c, then the blades 

 are obliged to stand vertically and assume a very narrow or linear form. 



* Climbing Plants, p. 74, figs. 3, 4. 



t For further details the reader is referred to The Origin of Plant Structures, 

 p. 104. 



Of course many terrestrial Monocotyledons have similar parallel-veined blades as 

 Grasses ; but there is reason to think such are referable to an original aquatic habit. 

 See " A Theoretical Origin of Endogens from Exogens, through Self-adaptation to an 

 Aquatic Habit," Journ. Linn. 8oc. t Bot. xxix. (1892), p. 485. 



