334 Knox: STEM OF IBERVILLEA SONORAE 
endocyclic groups. All of these small sieve-bundles without the 
leptome and such commissural sieve-tubes as serve to connect them 
with each other are transitory in their activity, and by the end of 
the elongation-period are obliterated. The obliteration follows 
the reverse of the order in which they appeared, so that the en- 
docyclic succeed the ectocyclic, and the commissures are the last to _ 
lose their identity. Fischer holds that the first sieve-tubes 
help to distribute the proteid substances to the young tissues of 
the stereome. Late they are reinforced in their office by the en- 
docyclic sieve-tubes. They develop from the procambial strand, 
while the endocyclic sieve-tubes are derived partly from the 
mother-tissue of the stereome and partly from the ground- 
parenchyma. Ectocyclic sieve-tubes come from the mother-cells 
of the collenchyma and stereome, and at first support the young 
collenchyma, which grows rapidly in the early stages. When it 
has thickened and reached its maturity the outer ectocyclic cells 
are obliterated. The remaining ectocyclic tubes in the chloren- 
chyma and all commissural cells come from the secondary meris- 
tems derived from the parenchyma-cells of the pericycle and cortex. 
The endocyclic sieve-tubes, as indicated above, nourish the de- 
veloping stereome-ring and when its permanent character is 
determined they are obliterated. The commissures are obliterated 
last of all, and the order of their degeneration is one of the strong 
reasons why Fischer concludes that they serve to withdraw the 
proteid substance from the endocyclic sieve-tubes and from each 
other into the leptome of the bundles. The office of the sieve- 
tubes of the bundles and the course of their obliteration needs no 
special comment; the development of the procambial strand and 
the support of the growing cambium is dependent upon their 
activity. The sieve-tubes outside of the stereome-ring are con- 
nected with those inside only at the nodes, so that the endodermis 
and the mechanical tissue are never broken by the commissures. 
The genera of the Cucurbitaceae are divided into classes upon 
the character of the supernumerary bundles, of which Maazmo- 
wiczta (Lbervillea) belongs to the Cyclanthera type. There are 
said to be present countless endocyclic sieve-tubes (FIGURE 4) united 
partly with each other and partly with the vascular bundle by 
very few commissures. The /éervillea at the Garden also possesses 
