THE ANATOMY OF A HYBRID EQUISETUM 
231 
cavity which is half way between the two. Further, the spores are 
always partially abortive, and the sporangium wall lacks spirals. 
Milde, in company with most other authorities, has concluded from 
this evidence that E. littorale is a hybrid between E. limostim and 
E. arvense. Exactly the same arguments are applicable to E. vari- 
egatum var. Jesupi, which must be a hybrid between E. variegatiim 
and E. hiemale. The presence of abortive spores, and absence of 
spirals in the walls of the sporangium seem to be reliable indications of 
hybridization, the present writer having found exactly the same con- 
ditions in a cross between Betula pumila and another birch. 
It seems obvious from this evidence that E. variegatum var. Jesiipi 
is clearly a hybrid, occupying an intermediate position between E. 
hiemale and E. variegatum . The question then arises whether it is the 
first filial generation of this cross. Before attempting to answer, 
there are various other facts to be considered. First, what is the 
relation of E. variegatum var. Jesupi to the European E. tmchyodon? 
The latter is likewise considered by Alexander Braun to be inter- 
mediate between E. hiemale and E. variegatum, and indeed there are 
similar arguments for its hybrid nature as for that of E. variegatum 
var. Jesupi. Both have abortive spores, and both have a combination 
of the characters of E. hiemale and E. variegatum, but the combination 
is different. Each has a loose sheath w^ith persistent teeth, but the 
bast of E. trachyodon is variable. According to Duval Jouve, the green 
parenchyma is interrupted under both ridges and furrows, while 
according to Milde, it is sometimes interrupted at only the ridges, and 
at others it is continjous around the whole stem. Moreover, while 
E. variegatum var. Jesupi has the endodermis characteristic of E. vari- 
gatum, E. trachyodon has that of E. hiemale, and further, E. irachyodo7t 
has the ridges of the stem biangulate, with a distinct carinal furrow. 
This last difi^erence is discounted by the fact that the European 
E. hiemale has bluntly biangulate ridges, while ours has them rounded. 
Secondly, what relation do these varieties bear to E. hiemale var. 
pumilum, which Eaton considers intermediate between E. hiemale 
and E. variegatum var. Jesupi, or to E. hiemale var. doelli, which Milde 
considers intermediate between E. hiemale and E. trachyodon? A 
similar difficulty exists if E. littorale is a hybrid between it. limosum 
and E. arvense: where do E. littorale var. arvensiforme and E. arvense 
var. diffusum belong? 
There are three possible answers: Either, as has been shown to be 
