Apr. fli. 1923 Occurrence and Significance of Phloem Necrosis 
241 
vantage of the natural great diversity of ecological conditions, almost 
any desired combination of soil, moisture, and climatic factors could be 
had, Otdy where the natural agencies were inadequate or where a special 
combination of factors seemed advantageous was artificial control of 
these factors resorted to. Material for the study of the effect of progres¬ 
sively increasing water supply was taken from a field a part of which had 
become inundated by seepage from an adjoining mesa. 
Plants suffering from an oversupply of water showed anatomical 
changes resemlJling those observed in foot diseases. The primary xylem 
and parts of the secondary elements of the wood were discolored and 
the lumen was filled with a brown, gummy deposit. The phloem part 
of the vascular tissue was commonly unaffected, and in cases where 
necrosis of the phloem was observed the material came from doubtful 
stock. The diseased phloem groups, which were occasionally observed 
in such material, did not take the lignin stain but showed the typical 
brownish discoloration which is characteristic of necrosis in general. 
A greatly reduced water supply, or alternate wetting and drying, 
produced a denser and more strongly lignified wood. It neither in¬ 
hibited nor accelerated the production of pathological changes. 
The effect of shading was found to be closely bound up with the photo¬ 
synthetic activity of the plant. Plants which grew to maturity in the 
shade of the river bottom vegetation were rank and slender. The 
xylem was greatly reduced and lignification of the cells less pronounced. 
The phloem remained normal throughout the life of the plants. 
Thus, qualitative changes in the phloem, as a result of extremes of 
environmental conditions, are uncommon. When present, the dis¬ 
eased grdups are localized and are connected with or are under the 
influence of heCrotic areas in adjacent tissues. The pathological changes 
in the phloem tissue consists in a browning, rarely a typical lignification 
of the cell walls. 
These observations, together with the fact that obliteration of the 
phloem, as a natural phenomenon in normal herbaceous plants, is dis¬ 
credited on theoretical grounds and disproved by extensive systematic 
studies; emphasized once more the fact that generalizations based on 
inadequate observations may lead us astray in the interpretation of true 
pathological changes. 
NECROSIS FOLLOWING EXTERNAL INJURIES 
By far the most common source of internal stem lesions is external 
injury produced by insects. The ensuing cellular changes may either 
constitute a direct response to the mechanical injury or be the result of 
iiritatioiis causing abnormal cell division and necrotic changes in the 
affected areas. 
The lesions become externally visible as abnormal swellings of the size 
of small pustules barely discernible to the naked eye (PI. i, A) or they are 
of the nature of large intumescences which may be seen in various stages 
of development. The oldest swellings are conspicuous by their brownish 
color and the fissured surface of the epidermis. The location of the 
lesions is the apical stem region and the young petiole. However, 
swellings may be found in any region regardless of the age and relative 
development of the organ. Concomitant with the formation of these 
intumescences is a change in the morphological development of the parts 
above the points of injury. The leaves are deformed and discolored. 
The leaflets fold along the midrib or roll in tubular fashion. The inter- 
