Carbohydrate Production in the Higher Plants 5 
had before been attributed to the water-relations of succulent plants 
to the arid soils in which they grew. In this paper on 'The Basis of 
Succulence ’ three Americans, MacDougal, Richards and Spoehr, who 
have worked more or less independently on succulents, have come to 
the agreed conclusion that succulence is due to a "metabolic com¬ 
plex favourable to pentose formation.” Pentoses lead on, by con¬ 
densation, to pentosan-mucilage with special water-holding pro¬ 
perties, and to other characteristics so marked in succulents. 
We may accept it then that here we have a deep-seated aberration 
of the ordinary carbohydrate flux tending more to the production of 
pentoses than usual. Here is one of those biochemical variations that 
we spoke of in opening, and it becomes of interest to note the dis¬ 
tribution of succulence among flowering plants. Certain families of 
plants are characteristically succulent, such as Cactaceae and Crassu- 
laceae, but we find succulent genera and groups of genera here and 
there among other families. Rochea, among the Compositae is a 
well-known example of an isolated succulent genus, but it possesses 
all the correlated physiological and anatomical properties that are 
found in the typical succulent families. 
It would seem that we must assume that the mutation in proto¬ 
plasmic constitution, which determines this direction of carbo- 
hydrate-flux, has occurred a number of times independently and that 
in some cases this has been followed by evolution of a whole group 
of allied genera and species, retaining the character, while in other 
cases only a single isolated form perpetuates the mutation. 
After this example let us turn to a biochemical point in which 
there is great variation through the range of flowering plants. This 
concerns the transition from the second stage, that of sugars, to the 
third stage, that of polysaccharides. It is well known that a number 
of plants form starch freely in their chloroplasts, as a so-called 
‘ temporary reserve * of the carbohydrate produced by photosynthesis, 
while many plants, on the other hand, form little or none. Mayer, 
in 1885, investigated all the plants in a botanic garden from this 
point of view and he found that this characteristic mostly holds 
true throughout a family of plants. He grouped the families examined 
into five classes, running from those that form starch very richly 
—Class I—to those that form none at all—Class V. Among Dico¬ 
tyledonous families, most come in Class II, Solanaceae and Papilion- 
aceae being alone in Class I, while Class V is represented only by 
Gentianaceae. When we come to Monocotyledonous families, it is 
