THE EMBRYOLOGY OF CARPINUS BETULUS. 41 
frequent branching of the tube. There is no reason to believe that every branch is 
supplied by a generative cell; on the contrary, some cases tend to prove that only two 
are present. 
In one case two egg-cells have been fertilized by two branches of the same tube; one 
branch, presumably representing the spur, having passed by the incompletely fused 
polars, leaving them unfertilized. 
In another ease the tube branches high up in the raphe; one branch passes to an 
egg-cell, the other fertilizes the definitive nucleus by means of a spur, and its tip is 
abortive. In two other slides, also, from one of which Pl. 6. fig, 11 is taken, the pollen- 
tube, after passing two or three large and vigorous definitive nuclei, only produces an 
abortive tip. The probability is that both male gametes have been liberated and have 
already fused with the adjacent nuclei. 
We have thus, we believe, demonstrated that while one male gamete remains in the 
tube and is carried up to the egg, the other male gamete is usually liberated free 
into the embryo-sac, though it may sometimes be used by its containing-branch to 
fertilize the eeg-cell of a second embryo-sac. The male gamete resembles in form and 
behaviour the majority of the vermiform nuclei that have been found in other plants, 
and is strikingly like that found in Lilium Martagon. 
The unique method of fertilization which we have found in Carpinus has evidently 
been acquired in correlation with chalazogamy ; and this elaborate adaptation to secure 
the fusion of the generative with the definitive nucleus testifies, as Prof. F. W. Oliver 
has pointed out, to the importance of the process of triple fusion to the plant. 
COMPARISON WITH CASUARINA. 
One outcome of this work is to demonstrate the similarity in the embryology of 
Carpinus and Casuarina. 
Besides the record which Treub * gave for the species Casuarina suberosa, 
C. Rumphiana, and C. glauca in 1891, Mr. Theodore Fryet has recently given an 
account of the development and contents of the embryo-sac of C. stricta; thus we 
have a fairly complete history of the processes which lead up to fertilization in that 
genus. For the sake of comparison we will state the salient points in the embryology 
cf Casuarina. us 
Secondary changes result in the ovary becoming unilocular, with two parietal ovules 
which are connected with the stylar cylinder or summit of the ovary by a bridge of 
tissue, as well as by the funicle. The ovule is anatropous and has two integuments, 
The chalaza is oblique, being directed to the raphe, and thus affords an easier approach 
for the pollen-tube, which enters the nucellus after descending the raphe. | 
'The sporogenous tissue is very bulky and gives rise to many fully equipped embryo- 
sacs, and later, in some species, to spirally thickened tracheides. The embryo-saes are at 
first isodiametrie, and contain in the earlier stages the eight nuclei characteristic of 
* Treub, Annales du Jardin Botanique de Buitenzorg, 1891. 
+ Botanical Gazette, vol. xxxvi, August 1903, pp. 101-113. 
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