429 



of the stems they were warts. In young spots a change in the bark par- 

 enchyma was noticed. The outer cells were no longer distinctly coUenchy- 

 matic and tangentially elongated but rather were broadened radially, thin- 

 walled, poor in chlorophyll and partially divided. Because of this structure, 

 the bark cells forced the cork tissue outward, causing whitish blisters or 

 warts. 



In Opuntia and Phyllocactus, the second variety of cork outgrowth is 

 prevalent and is characterized by the formation of depressed places or by 

 total perforation. Fig. 70 of a Phyllocactus illustrates both forms of cork 

 excrescence. On the under side we see wart-like convexities, on the upper 

 bide the beginnings of perforation. 



A cross-section of the fiat stem shows the fleshy bark beyond the vas- 

 cular l)undle. In healthy places the 

 bark is filled with starch (sf) and con- 

 tains numerous slime cells (s). cal- 

 cium oxalate crystals and glands ( 0) . 

 when the wart begins to form, the bark 

 |)arenchvma, by utilizing the starch, 

 stretches. di\ ides and pushes out the 

 epidermis. The peripheral tissues (i) , 

 poor in contents, begin to die and a 

 la}'er of flat cork cells (/) separates 

 the dead tissue containing many inter- 

 cellular spaces filled with air from the 

 still li\ing succulent tissue. At this 

 point the progress of the disease stops 

 and the stem seems covered with dry 

 paper-like spots. If. however, there 

 is no further removal of starch nor 

 stretching of the bark parenchyma, and 

 large particles die, the upper surface 



of the dead tissue finally ruptures, forming holes (/) which gradually be- 

 come more and more depressed v.hile the flattened cork cells (t) are 

 constantly formed, growing inward. At r the bark changed, giving rise to 

 the cork formation. There the change occurred earliest and most intensively 

 and advanced rapidly into the interior of the leaf. 



The process of cork formation is in itself a normal process in cacti 

 when the stems reach a certain age. At the base of older stems there ma}- 

 !)e seen a formation of bark as in trees. The pathological feature is the for- 

 mation of flat cork layers in the younger parts, at the expense of the bark. 

 The cause may be found in the formation of tissue centers in the bark m 

 which the cells elongate, while the starch breaks down and the cell contents 

 are gradually impoverished. 



Fig. 71 shows the first change in the tissues, in the formation of bark 

 types of cork excrescence. This illustrates a piece of bark from Phyllo- 



Fig-. 71. First stage of a cork 

 excrescence in Phyllocactus. 



