44 Sargant. — T heory of the Origin of Monocotyledons 
and I. sibiricd) or triarch (I. Boissieri). The root of Iris sp . 
appears diarch when first formed, but very soon becomes 
tetrarch, and the whole process of transition, so far as it could 
be followed in the rather old seedlings, recalled that of 
Chlorogalum (p. 29). 
Freesia sp. (garden variety). The cotyledon contains three 
bundles in its lower part or sheath, and the central one appears 
double just above the first node. One plumular trace at least 
takes part in the transition. The root is tetrarch. 
Aroideae. 
Three species representing three genera have been examined 
from this family ; Arum maculatum very thoroughly, the 
others in less detail. 
The structure — internal and external — of the Arum macu- 
latum seedling has been described elsewhere (36). The seed 
ripened in the summer may germinate before winter, or remain 
dormant until the spring. In either case no part of it appears 
above ground until the second spring after the seed has been 
sown. 
The apex of the cotyledon remains within the seed as 
a sucking organ, while the lower part is transformed into a 
cylinder which sheathes the young stem-bud and is terminated 
by the hypocotyl. This begins to be thickened in the early 
stages of germination, and it swells into a tuber as the stores 
of food are transferred to it from the endosperm through the 
bundles which run down the cotyledonary tube. By the end 
of the summer following the sowing of the seed, the whole 
cotyledon, having emptied the seed of its food-supplies, is 
detached from the tuber. The stem-bud is exposed by the 
removal of the tube which has hitherto surrounded it, and in 
the following spring the first green leaf pushes through 
the soil. 
The vascular system of the cotyledon has been profoundly 
modified by its peculiar habit. Of its five bundles the central 
one is larger than the others, but it never appears double. 
The plumular traces are inserted at the first node on those 
