Actinostemma biglandulosa. 643 
since the cells of the fundamental tissue adjacent to it are 
active and make way for its increase. In the case of the 
large inner bundles this dilatation of the fundamental tissue 
surrounding them causes the compression of the pith and of 
the more central portions of the medullary rays. 
As in other Cucurbitaceae 1 secondary thickening is at 
first confined to the bundles: the interfascicular cambium 
appears late and forms only parenchyma. The tension of 
the tissues thus set up ruptures the ring of sclerenchyma at 
points between the bundles, and cells insert themselves into 
the gaps and subsequently divide. 
The bundles remain open throughout the life of the plant. 
While the normal cambium continues to function, additions 
are made to the medullary phloem by means of the medullary 
cambium. The medullary phloem in Actinostemma is small 
in amount as compared with the external phloem, and its 
sieve-tube elements have only about half the diameter. 
Later on, phloem develops at the sides of the wood, which 
thus comes to be surrounded on all sides by that tissue 2 , as 
occurs in other Cucurbitaceae. 
In Actinostemma the medullary phloem does not *' accom¬ 
pany the leaf-traces on their exit into the leaf’ (contrast 
Weiss). Transverse sections of the older petiole showed 
collateral bundles. The medullary phloem accompanies the 
cotyledonary traces to their lower ends in the hypocotyl, 
where it stops. The tetrarch or triarch root has no pith 
and consequently no internal phloem. There is abundance of 
thin-walled parenchyma in the secondary wood of the root. 
A search was made for interxylary phloem, such as is de¬ 
scribed by Scott and Brebner (op. cit.) for the root of Thla - 
diantha dubia , but none was found. 
1 De Bary, Comparative Anatomy, English edn., p. 328. 
8 De Bary, Comparative Anatomy. 
