DEPARTMENT OF BOTANICAL RESEARCH. 69 



The Daily Course of Growth in Two Types of Fruits, by D. T. MacDougal. 



Two different types of organs or shoots with respect to the variations 

 in the water-content and dry weight are recognizable. The commoner 

 types of woody stems, thin leaves, and the organs of the greater number 

 of the higher plants undergo a development which terminates in a 

 mature stage in which the proportion of solid material is very much 

 higher than that found in younger material. A parallel procedure is 

 the prevalent one in the tissues of the higher animals. 



Etiolated plants furnish examples of growth \\dth a diminished 

 increase in dry weight, but chief interest attaches to plants which nor- 

 mally show such action, and the most striking illustrations are to be 

 furnished by the organs of succulent plants and by fruits. The relative 

 amounts of solid material in the flattened joints of Opuntia do not 

 increase with the course of development toward maturity, and joints 

 which have reached full size may contain over 91 per cent of water. 

 Secondary thickening, especially that which results from branching and 

 the development of additional fibro- vascular tissue, may cause an added 

 amount to be formed. The proportion of dried material and water in 

 the leaves of Mesembryanthemum does not vary greatly with age. 

 Extended discussions of the growth of these succulents have been given 

 in previous publications of this Department. Following a full recog- 

 nition of the two types of growth in 1918, a final series of experiments 

 was arranged in which the enlargement of fruits with increasing dry 

 weights and with small dry weights should be measured. The walnut 

 was taken to represent a structure with accumulating solid matter 

 and the tomato for the other type. 



The walnut consists of a thick fleshy exocarp and a heavy endocarp 

 which finally becomes hard and bony wdth the deposition of anhy- 

 drous wall-material. The inclosed embryo also accumulates a large 

 amount of condensed food-material. The tomato is a large globose 

 berry in which deposition and thickening is confined to the small, 

 hard seeds. The greater part of the fruit is a fleshy, watery pulp, which 

 becomes more highly hydrated as progress is made toward maturity. 



The measurement of walnuts, beginning at a stage when they had 

 a diameter of but 3 mm. and extending to maturity, showed that these 

 nuts displayed a daily variation corresponding with the balance between 

 transpiration and absorption of such character that enlargement 

 began after noon and continued until sunrise, at which time a retarda- 

 tion or shrinkage set in, which continued until midday. It is notable 

 that the young growing nuts exuded water when cut into, and were so 

 nearly saturated that they showed but slight swelling in water, drew 

 their supply from stems with such a water deficit that a swelhng of 10 

 per cent was displayed, and an amount of water equivalent to 25 per 

 cent of their volume might be taken up when immersed. Such absorp- 



