THE ANGIOSPERMAE 1527 



abscission layer in the pedicel and thus permit fruit development to go on 

 without pollination or other stimulation, such fruits may also be aided in 

 development by growth-substances applied artificially. The two seedless 

 grapes mentioned above, if treated in flower or in the young fruiting stage 

 with beta-naphthoxy-propionic acid yield considerably bigger fruits. 



It is known that pollens contain growth- promoting substances and we 

 may conclude that in normal fruit production the first stimulus to fruit 

 formation comes from this source. Several of the above examples do 

 suggest, however, that developing seeds also exercise an influence upon 

 fruit development, which may be a continuing influence, necessary for the 

 growth of the fruit to its full size and character. Indeed it has frequently 

 been remarked that if one carpel of a syncarpous fruit is sterile that part 

 of the fruit is under-developed compared with the rest. 



Nearly all the cases of parthenocarpy which we have mentioned refer 

 to fleshy fruits, but dry fruits may also be parthenocarpic, e.g., in Acer 

 Hesperis, Pisum, Nicotiana and Papaver. 



The extent ot the development which takes place in the ovary during 

 fruit formation is extremely variable. In some species there is little enlarge- 

 ment and there is no new cell formation. In others there may be relatively 

 enormous growth, as in Cucurbitaceae, so that it is difficult to recognize in 

 the mature fruit, weighing thirty or forty pounds, the product of the tiny 

 ovary of the flower. Such growth is accompanied by great multiplication 

 as well as enlargement of cells. From being only a few cells in thickness, 

 the fruit wall may come to possess forty or fifty layers of cells. 



The time taken for fruit development is likewise very variable, though 

 there is a general proportionality between size and ripening period. Many 

 herbs, especially weeds of open ground, mature their fruits in one or two 

 weeks and may therefore pass through several generations in one growing 

 season. The great majority of plants take several months, but are within 

 the limits of a single year. The large fruits of some Palms take at least a 

 full year and the great Coco de mer of the Seychelles, Lodoicea, may take 

 ten years. Many Orchid fruits are also slow to ripen and may take two 

 years or more. They are often massive and contain millions of seeds, so 

 that the accumulation of the necessary quantity of food materials may be 

 the retarding factor. 



The ripened ovary wall becomes the coat of the fruit and is then called 

 the pericarp. The classification of true, ovarial fruits, depends in the first 

 place on the consistency of the pericarp, which may be either dry or 

 fleshy, and secondarily on whether the fruit wall does or does not open to 

 release the seeds. 



I. Dry Fruits 



I. hidehiscent Dry Fruits. The pericarp does not open and the fruit 

 with the seed is shed from the plant as a unit (Fig. 1391). 



(a) Akene or Achene. The product of a single, uniovulate 



Q* 



