288 The Phenomena of Morphogenesis 



There are various sorts of amorphous structures which are not easy to 

 distinguish from each other, but a number of categories may be recog- 

 nized, such as intumescences, callus, tumors, and galls. The terms "gall" 

 and "tumor" have no very precise meanings, but a gall is most commonly 

 regarded as an anomalous growth due to an attack by a parasite and a 

 tumor as one which results from other causes, though there are many 

 exceptions to these definitions. 



Intumescences. The simplest sort of amorphous abnormal growth is one 

 in which a group of cells at the surface of an organ expands into a wart 

 or pustule. These are termed intumescences (Sorauer, 1899) and result 

 from various causes (Wallace, 1928). Sometimes, as on the leaf of cab- 

 bage (Von Schrenk, 1905), they are groups of watery (hyperhydric) cells, 

 swollen by excessive absorption of water resulting from contact with 

 spray materials or other substances. In woody plants intumescences 

 usually are formed by proliferation of cork cells, sometimes from lenticels 

 and sometimes elsewhere. Such intumescences involve an increase in the 

 cell number (hyperplasia) as contrasted with an increase in cell size 

 (hypertrophy) as in the cabbage leaf. They may result from exposure to 

 ethylene gas and other substances. Intumescences often resemble natu- 

 rally occurring corky spots on certain plants, especially at lenticels. 



Callus. As a result of wounding, a layer of cork cells is usually produced 

 over the wound surface through the action of wound hormones (p. 402). 

 This perhaps is not to be called "abnormal" tissue in the ordinary sense, 

 since it is very common and indeed accompanies bark formation in trees, 

 where new cork layers cover the breaks resulting from expansion of the 

 axis. In many cases, however, these do not occur in the intact, uninjured 

 plant and may best be included among abnormal structures. 



In cases of more serious wounds, as where a cutting is removed for root- 

 ing, something more complex than a few layers of cork cells is formed 

 at the surface. Here often develops callus, an amorphous mass of rather 

 large-celled, loosely arranged parenchymatous tissue, produced by cell 

 division in the ground tissue or more commonly from cambium (Fig. 

 11-9). Its elements show relatively little differentiation, but there may be 

 some meristematic growth near the surface. For its nutrition, callus de- 

 pends on food from the normal tissues beneath it. 



A callus may reach considerable size but its mass has no definite form 

 and there is little morphogenetic control over its growth. In its later 

 stages, callus may undergo various types of development depending on 

 the kind of plant, the location of the wound, and the external conditions. 

 Often some differentiation appears in it, and cells are produced resem- 

 bling those of normal tissue. Isolated nests of single cells or groups of 

 cells may develop into tracheid-like elements, usually with reticulate pit- 

 ting. Sclereids are formed in the same way. These irregular nests have a 



