Translocation in Plant Tissues. 
307 
have localized meristems and grow fairly rapidly, are histologically remark- 
ably similar to those of Angiosperms, and must provide the same facilities 
for translocation. Some confirmatory experimental evidence in support of 
this contention has been obtained by the osazone method , 1 but the work is 
not yet complete. 
In such plants as Mosses, which are never very large, and modern 
Lycopodiales, which are microphyllous with closely arranged leaves, and 
grow comparatively slowly, demands for vigorous sugar transport are less 
urgent than in the higher plants ; this is correlated with a less advanced 
development of special conducting cells. 
True sieve- tubes have not been demonstrated in the Mosses, but in the 
larger types there are elements called ‘ leptoids 5 2 which have been assumed 
to provide conducting channels for assimilates. 
Histological examination 3 of the vegetative portions of Polytrichum 
has revealed the fact that the connecting-threads in the leptoids are very 
numerous indeed, though they show no sign either of being bored out to 
give slime-strings, or of being enclosed in tubules of callose. 
In the terminal walls of the leptoids the connecting-threads are far more 
numerous than in any other type of cell in the plant. These walls are often 
very oblique and usually bag-shaped, the end of a cell often giving the 
effect of being slightly invaginated. In this way the area of the wall is 
even more increased than it would be if it were simply an obliquely placed 
plane. 
While decisive evidence in support of the view that leptoids conduct 
assimilates has not been adduced, yet in the light of the considerations above 
it is clear that their histological features render them very suitable for such 
a function. The end walls are structurally of a type between the walls of 
ordinary parenchyma and true sieve-plates, and evidently suffice to meet 
requirements. 
In the modern Lycopodiales the leaf leptome is of a simple type. In 
most cases numerous sieve-tubes occur in the stem, but they are not provided 
with companion-cells like those of the Angiosperms. 
On the other hand, almost perfectly preserved examples of the fossil 
representatives of the group have been described as showing no elements 
which could appropriately be called sieve-tubes , 4 and hence are devoid of 
secondary phloem, although the xylem underwent considerable secondary 
increase. 
In this connexion the following considerations may be of interest. 
1 Mangham (1911 (2)). 2 Tansley and Chick (1901), 
8 Hume (1913). In correspondence also Miss Hume has kindly supplied certain details of her 
work on Polytrichum . 
* Seward (1902). In a recent letter Professor Seward has stated that he has no reason for 
changing the opinions expressed in the paper referred to. 
