PT 
PREFACE. xxxiii 
embryonary evolution, or certainly of imperfect germi- 
nating powers, this though liable to some excep- 
tions appears true enough; it explains why* albu- 
men is so common among  monocotyledones. [ts 
presence is incompatible with the existence of a high- 
ly developed, plumula, a remarkable fact, and one 
particularly evident in some Aroidea which contrary 
to the habit of the order, have no albumen, the defi- 
ciency being supplied by a high degree of develop- 
ment of the Plumule. 
The next point to be enquired into, is the relative du- 
ration of these coats, which are almost invariably 4 in 
number. The only instance, I as yet know of to the 
contrary occurring in Xanthium, in which there is a 
‘membranous sac, of excessive tenuity which lines the 
embryonary sac. These four coats the existence of 
which is possible are, the outer and inner integuments, 
the nucleary membrana, and the embryonary sac; of 
these the Ist is always permanent, remaining in the 
shape of the testa, as is likewise the embryonary sac. 
Traces of the two others are frequenily imperceptible, 
and it may be expressed as a general rule;-that the in- 
ner integument of the ovulum has no existence in the- 
seed. This is well expressed by Dr. Lindley in his 
Introduction to Botany, p 184. 
The nucleary membrane frequently exists, it is 
known from either of the preceding by being imper- 
_* Note—Mr. Brown long ago remarked, that albumen was the 
type of Monocotyledons, bat no one has I believe remarked, that 
the proportions in favor of albumen among Dicot. — are 
nearly equal to those of Monocotyledons. . 
