ANOMALOUS LEAVES AND PETIOLES. 



85 



occurrence taking place after they have been developed; but is in- 

 timately connected with the general law, in accordance with which the 

 parts of the plants are formed. 



161. Stipules are sometimes large, envelop- 

 ing the leaves in the young state, and falling 

 off in the progress of growth, as in Ficus, 

 Magnolia, and Potamogeton; at other tunes 

 they are so minute as to be scarcely distin- 

 guishable without the aid of a lens, and so 

 fugaceous as to be visible only in the very- 

 young state of the leaf. In grasses, the sheath 

 or sheathing petiole (fig. 194^rv) has a prolon- 

 gation or folding of the epidermis * at its upper 

 part, distinct from the leaf, to which the name of 

 ligule (ligula, a small slip) has been given (fig. 

 194 g I). Some consider it as equivalent to a 

 stipule. It is either long or short, acute or 

 blunt, entire or divided, and thus gives rise to 

 various characters. At the base of the leaf- 

 lets or foliola of a compound leaf, small stipules 

 are occasionally produced, to which some have 

 given the name of stipels. 



Anomalous Forms of Leaves and Petioles. 



162. Variations in the structure and forms of leaves and leaf- 

 stalks are produced by the increased development of cellular tissue, by 

 the abortion or degeneration of parts, by the multiplication or repeti- 

 tion of parts, and by adhesion. When cellular tissue is developed to 

 a great extent, leaves become succulent, and occasionally assume a 

 crisp or curled appearance. Such changes take place naturally, but 

 they are often increased by the art of the gardener; and the object of 

 many horticultural operations is to increase the bulk and succulence of 

 leaves. It is in this way that Cabbages and Greens are rendered more 

 delicate and nutritious. 



163. In some plants true leaves are not produced, their place being 

 occupied by dilated petioles or phyllodia (^[ 157), or by stipules 

 (^f 159). In other instances scales are formed instead of leaves, as in 

 Orobanche, Lathrsea, and young Asparagus (fig. 110 I). Divisions 

 take place in leaves when there is a multiplication of their parts; 

 and a union of two or more leaves, or of parts of leaves, occurs in many 



* See Deduplieation, under the head of Corolla. 



Fig. 194. Portion of a leaf of Phalaris arundinacea, one of the grasses. /, Laminar merithal 

 or blade of the leaf, with straight parallel venation, g r. Vaginal, or sheathing portion repre- 

 senting the petif-if, ending in a membranous process or ligule, g I. 



