608 THE AMERICAN NATURALIST. : [Vor XXXIL 
archegoniates, is that the sporogenous structures of the sporophyte 
are older than the vegetative ones. 
The discussion of the division of labor and development of special 
organs in the Thallophytes is treated clearly and interestingly, but 
offers nothing especially new. 
The section dealing with the question of cohesion and reduction 
of parts is clearly presented, and Goebel, like most students who 
have made a practical study of developmental morphology, recog- 
nizes the absurdity of assuming that all simple flowers such as many 
apetalous Dicotyledons and the lower Monocotyledons like the 
Aracez and Naiadacez are necessarily reduced from some forms 
with more complex flowers — a relic of the old metaphysical notion 
of a “typical flower” to which all other types must be made to 
conform. 
The second division of the volume deals with the question of 
symmetry in the plant-body. It is treated at length and the author 
brings up many interesting points, especially those dealing with the 
causes and significance of bilaterality or dorsi-ventral symmetry in 
shoots and leaves, as well as zygomorphy in flowers. In regard to 
the latter point, he concludes that we are not much nearer to under- 
standing the mechanism by which they have been produced than 
were Sprengel and De Candolle. All we know is that they are in 
most cases associated with cross-fertilization, and that zygomorphic 
flowers are always lateral in origin. 
The most interesting part of the book is the portion dealing with 
the changes in the character of the organs of the plant, especially 
the leaves, as the plant develops from its earlier stages to maturity. 
Goebel has already published several very important contributions to 
this most interesting subject, but he adds here a good deal that has 
not before appeared, and at the same time includes a summary of the 
more important results of his earlier investigations, especially with 
regard to the changes in the form of leaves and the significance of 
these early leaf-forms. Perhaps the most important of the new types 
brought forward here is that of certain tropical Aroids, especially 
some of the climbing forms. These striking plants are very con- 
spicuous in the American tropics. Goebel made a special study of 
some of these and found that in their earlier stages of growth they 
had simple, sessile leaves, closely overlapping and completely con- 
cealing the stem. The flowering shoots, however, lose the dorsi- 
ventral character, and the much larger and often variously cut leaves, 
£g., Philodendron, are borne upon long petioles. It appears that 
