Physiological Studies in Plant Anatomy 39 
might be due to the greater stretching of the less durable cell walls 
under the pressure of the turgid pith and was naturally puzzled how 
to explain the lack of extension of the leaf. Rauwenhoff(2i) agrees 
as to the lack of thickening of the cell wall in the etiolated stem, but 
considers that the cortex contributes to the expansive pressure which 
stretches the walls and that negative geotropism also plays a part 
now that it is not inhibited by counteracting heliotropic stimuli. In 
the light of observations in the first part of this paper it is clear that 
the excessive elongation in the broad bean type of etiolation must be 
traced to more definite structural causes and in part to the presence 
of a functional primary endodermis confining the growth to an 
extension in length as the result of the activity of an apical meristem, 
whilst structural developments in the cortical region are cut down 
to a minimum. 
The presence of an endodermis has been noted by Costantin(2) in 
the case of certain underground stems as in Rubus (loc. cit. p. 24). 
In Ricinus he finds the endodermis continuous in subterranean stems, 
interrupted in etiolated stems, and absent in those grown in the light 
From Costantin’s data it seems clear that many underground stems, 
such as those of Anemone nemorosa and Mercurialis perennis, have 
a primary endodermis, whilst the aerial shoots are lacking in this 
respect. In other cases the buried rhizome possesses a secondary 
endodermis, e.g. Galium cruciatum, Achillea Millefolium. Costantin, 
however, does not indicate in any way the structural results following 
from the presence of the endodermis, and indeed seems to suggest 
(loc. cit. p. 47) that in the aerial stem the endodermis has passed on 
to a further stage, whilst the functional primary stage or later suber- 
ised stage persists underground, whereas the important fact is that 
the endodermis never has been present as a functional layer within 
the aerial stem. Costantin’s observations seem to indicate in many 
further cases the presence of an endodermis below ground and its 
absence above. Thus in Genista sagittalis, and many other Legu- 
minosae, he notes the change in outline of the stem in cross section 
from round in the underground region to angled or winged above, 
which coincides exactly with our own observations on the broad bean 
and potato. He also reports the suggestive fact that in many under¬ 
ground stems the cortex is exfoliated as the result of the activity of 
a deep-seated periderm. The suggestion that this periderm is peri- 
cyclic and arises within a functional endodermis is therefore irresistible. 
In the case of species of Potentilla this has been found to be true, 
and other examples are being examined as opportunity occurs. 
