Compound Rays in the Lower Dicotyledons. 227 
number of intermediate steps of transformation exist between these 
characteristic ones picked to demonstrate the main steps of the compound- 
ing process. 
The evidence derived from the study of the development of ray 
structures in the life-history of the individual alder plant affords as con- 
clusive a demonstration of the mode of development of aggregate or com- 
pound rays as does the comparative study of ray structures in the mature 
wood of distinct species of the genus Alnus . In the early stages of the 
development of the plant primitive stages of compounding and fusion occur, 
and with subsequent growth more highly perfected stages of aggregation 
are progressively developed. Thus in the first-formed annual rings of the 
seedling plants of certain alders non-aggregated uniseriate rays occur 
exclusively, but in subsequently formed rings radiating bands of tissue 
arise, which are characterized by congeries of uniseriate rays and by the 
absence of vessels. In the radial extension of these aggregate rays during 
many years’ growth progressively higher stages of compounding occur, 
such as have been illustrated in Plates XV and XVI. As in recent years, 
through the researches of Strasburger, Goebel. Jeffrey, Jackson, and others, 
the importance of seedling plants in the recapitulation of ancestral characters 
has become a firmly established canon of comparative anatomy, the 
evidence afforded by the examination of these alder seedlings is an impor- 
tant guide to the phylogenetic history of the development of aggregate or 
compound rays. 
It is of interest to compare the condition just described in the genus 
Ahms with that which occurs in oak. Upon the examination of material 
of a large number of American oaks the interesting fact was discovered, 
that in most species of American Live Oaks the so-called ‘ false ray ’ occurs 
in a more or less advanced stage of development towards the large com- 
pound ray, characteristic of oaks with deciduous foliage. In Figs. 12 and 13 
may be seen tangential and transverse sections of Quercus virginiana , Mill. 
( Q . virens , Ait.), the common Live Oak of the southern United States, which 
has reached a compounding stage similar to that seen in Alnus rubra in 
Fig. 5. Among other Live Oaks stages may be found in which fusion is 
even less well developed, and many advanced stages occur in the trans- 
formation of the aggregating into the broad, homogeneous type of ray. 
Strong evidence of the development of the large rays by a compounding 
process also exists in the American White and Red Oaks. Included fibres 
and wood parenchyma cells, vestiges of a compounding process, occur in 
the rays and display good evidence of their gradual transformation into ray 
parenchyma. A striking piece of evidence in this connexion is the fact that, 
whereas crystal cells are found abundantly in the large rays and in the 
wood parenchyma of many species of oak, these crystals are absent or 
extremely rare in the uniseriate rays. The presence of crystals in the large 
Q ^ 
