92 
ADA HAYDEN 
Jost, by removing the leaves of seedlings of Phaseolus multiflorus, Helian- 
thus annuus, and Vicia Faba, found that the vascular bundles supplying 
the amputated leaves remained rudimentary. There is evidence of a proc- 
ess of adaptive self-regulation. 
Kohl has demonstrated that the water-conducting system may be re- 
duced by growing the plants in a moist atmosphere, and Schenck has dem- 
onstrated the reduction of vascular bundles of Cardamine pratensis by grow- 
ing the terrestrial plant in water. Plants which are naturally amphibious 
exhibit a similar character. 
The endodermal layer has been a subject of considerable experiment 
and study. In some plants the cells of this layer have thickened inner 
walls, and in some the walls are of uniform thickness. Schwendener has 
shown that the endodermis is impermeable except in spots where it acts 
as side sluices in a system of irrigating canals, the main channels of which 
are represented by the vessels. Greatly thickened endodermal layers are 
found in marsh plants which live in places likely to dry up periodically. 
This is true of some plants growing in dry regions. 
Gaseous exchange is slow under aquatic conditions and is accelerated by 
the presence of air spaces. Aerenchyma is also found in marsh plants sub- 
ject to submerging. 
These facts show that the relative distribution and proportion of me- 
chanical tissue, parenchyma, and conductive cells are of considerable sig- 
nificance with reference to adaptation in the economy of plant tissues. 
Description of Subterranean Organs 
Typhaceae 
Typha latifolia L. 
Habitat: Alluvial basin; marsh. 
Gross structure: Long, stout, horizontal rootstocks; 6-8 in. deep; soft and spongy, 
origin from a thick stem base; long, slender; straight roots, at notes, perennial 
(Plate XV, fig. d). 
Histology of rhizome (Plate XVI, fig. i): 
Primary cortex: Radius of cortical area }4 radius of rhizome; hypodermis, thin- 
walled parenchyma ; next a zone of rounded-compact parenchyma in which vas- 
cular bundles are originating; zone of aerenchyma in which vascular bundles 
have enlarged ; bundles surrounded by woody sheath. Endodermis pronounced, 
with thick inner wall. 
Stele: Composed of one row of large, woody bundles surrounding the pith cyl- 
inder of thin-walled aerenchyma, among which are scattered smaller vascular 
bundles with compact, thick, woody sheath. 
Summary: Air space prominent; parenchyma well developed; vascular tissue fairly 
abundant; slight mechanical tissue, found only at sheath for bundles; vascular 
strands act as mechanical tissue in cortex. 
Histology of root (Plate XVI, fig. 2): 
Primary cortex: Composed of 4-sided to oval parenchyma cells; air space small; 
no mechanical tissue. 
Stele: Radius 4/5 that of the root; conducting vessels fairly prominent, surrounded by 
parenchyma. 
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