ANOMALOUS LEAVES AND PETIOLES. 



99 



stipules is not an accidental occurrence taking place after they have 

 been developed, but is intimately connected with the general law, in 

 accordance with which the parts of the plants are formed. 



Stipules are sometimes large, enveloping the 

 leaves in the young state, and falling off in the 

 progress of growth, as in Picus, Magnolia, and 

 Potamogeton ; at other times they are so minute 

 as to be scarcely distinguishable without the aid 

 of a lens, and so fugaceous as to be visible only 

 in the very young state of the leaf. They may 

 assume a hard and spiny character as in Eobinia 

 pseudacacia, or may be cirrose, as in Smilax, 

 where each stipule is represented by a tendril; 

 while in Cucurbitace^ there is only one cirrose 

 stipule. In grasses the sheath or sheathing 

 petiole (fig. 210 g v) has a prolongation or fold- 

 ing of the epidermis at its upper part, distinct 

 from the leaf, to which the name of h'ffule (ligula, 

 a small slip) has been given (fig. 210 y 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 leaflets or foliola of a com- 

 pound leaf, small stipules are occasionally pro- 

 duced, to which some have given the name of itipels. 



Pig. 210. 



Anomalous Forms of Leaves and Petioles. 



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 repetition 

 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 Savoys are rendered 

 more delicate and nutritious. 



In some plants true leaves are not produced, their place being occu- 

 pied by dilated petioles or phyllodia (p. 96), or by stipules (p. 97). 

 In other instances scales are formed instead of leaves, as in Orobanche, 

 Lathrsea, and young Asparagus (fig. 129 I). Divisions take place in 



;Kg. 210. Portion of a leaf of Phalaris anindinacea, one of the grasses. /, Laminar 

 merithal or blade of the leaf, with straight parallel venation, g v, Vaginal, or sheathing 

 portion, representing the petiole, ending in a membranous process or ligule, g I, 



