4s I 74 s ipuctdral and Ph isiologcalB Boteny 
moment in which the rays of light fall upon them ; and in diffused 
daylight after a short time. Plants grown in the aaa and- therefore” 
etiolated, develope chlorophyll first when exposed to light, and. then 
starch in the chlorophyll-grains. In the dark, starch is transformed into — 4 
other nearly related but soluble substances, such as sugar, dextrine, &C., 
which become distributed through the whole plant, and is again fixed in 
the form of starch as a reserve-material at particular places, as in the grains 
of cereals, the tubers of the potato, &c. Whether or to what extent the 
nitrogenous constituents assist in these processes is still unknown. 
The processes of life cannot be conceived as going on, 
even in unicellular plants, without a continual movement of — 
those substances which are necessary to their life. In the 
most simply organised plants these processes are simple, 
but become more complicated as we ascend in the scale. 
The direction in which the assimilated substances move 
varies according to the vital needs of the plant. They can 
be transported from the places where they are formed 
directly to those where they are required, or to reservoirs of 
veserve-matertal, where they are stored up until they are 
again carried to the parts where they are consumed. Hence 
perennial plants have their chief reservoirs in their stems, 
rhizomes, bulbs, or tubers; while those which do not bear 
fruit more than once have the greater part of their assimi- 
lated substances stored up in their seeds. 
It may be taken as nearly certain that in plants higher 
in the scale than Mosses, in most if not in all cases, the 
transport of the assimilated nutrient sap takes place through 
two forms of tissue. The soft-bast is chiefly, though not 
exclusively, the conducting tissue of the albuminous muci- 
laginous alkaline compounds ; while the parenchyma, both 
of the cortex and of the pith, is the chief agent in the con- 
duction of non-nitrogenous substances, such as starch, sugar, 
inuline, oils, and acids. ‘The laticiferous and the vesicular 
vessels also contain reserve-materials of a different kind, 
which can be made use of when required ; but the move-. 
ment of the fluids contained in them is more slngeui than | 
.g ina small- celled tissue. 
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