409 
Physiology of Pulpy Fi'iiits. 
the pressure caused by mutual contact does not exist. The 
cells, two or three deep, which lie internally to the thin-walled 
elements above mentioned, undergo a remarkable change of 
shape shortly before the fruit reaches maturity. They suffer 
an enormous extension in the radial direction, so much so that 
thej r can be compared to nothing so well as to cylinders whose 
ends are rounded. Intercellular spaces make their appearance 
during this alteration which separates one cylindrical row from 
the others around it, and serve to bring out the striking indi- 
viduality of these cells into yet sharper relief. It is owing to 
the presence of these cells, whose thin walls bear so small a 
proportion to the bulk of fluid which they contain, that the 
interior of the fruit appears to be filled with nothing but juice. 
If the walls had been a little thicker, and the cells greater in 
number and consequently smaller, a fruit would have been 
produced which would have occupied a position intermediate 
between such forms as the blackberry and the more fleshy 
types which are represented by members of the other end of 
the series. 
The cylindrical cells pass quite abruptly into a tissue com- 
posed of small and crowded cells, which forms an ill-defined 
layer outside the endocarp. It is not easy to say whether 
they should also be referred to the mesocarp, and yet they 
certainly do not belong to the endocarp, but they form rather 
a neutral ground between these layers, and serve merely to 
establish continuity between the inner and middle zones of 
the fruit. 
It is manifest that a considerable difference exists, both as 
regards the origin and mode of development of the pulp in 
the two cases we have hitherto considered ; for whilst in Heaera , 
in addition to the carpels, the raised up portion of the torus 
also enters in part into its formation, in Rubus the whole is 
formed from a limited portion of the tissue of the carpels. 
Again, whilst in Hedera all the characters, apart from those 
which are the result of secretional activity, could be predicted 
frorn the structure of the bud, in Rubus it is not until the ap- 
proach of maturity that the most striking features of the fruit 
