VII GROWTH OF SPECIFIC ORGANS 807 



is a high concentration compared to those used in the elongation studies above; 

 however, the isolated segments used were not elongating in these experiments. 



The initiation of laterals evidently depends on periclinal cell divisions occurring 

 in a specific layer, the pericycle, and the action of auxin is probably exerted in 

 stimulating these divisions. In Torrey's work the laterals were counted as un- 

 elongated initials, still within the main root, so that the role of elongation was 

 essentially excluded. The process thus offers another instance of the effect of auxin 

 on cell division. Elongation of the formed laterals, just like elongation of the pri- 

 mary root, would be strongly inhibited by the auxin, but since 3 days suffices for 

 initiation, the roots can be removed from the auxin solution and the laterals will 

 then grow out. In these respects the formation of lateral roots on a root is compara- 

 ble to the formation of roots on stem cuttings, which is also promoted by auxin 

 and assisted by various nitrogenous compounds (p. 771). 



(c) The growth of fruits 



In this limited space it would be quite impossible to discuss the entire topic of 

 fruit growth. Both the physiology (Nitsch, 1952, 1953; Leopold, 1955, Chaps. 

 II to 13) and the anatomy (Essau, 1953 Chap. 19) have been well reviewed. 

 Several short reviews were included also in the Wisconsin Symposium (Skoog, 

 ed., 1951). Only a few points will be made here. 



In general, the fruit consists of the swollen ovary wall, receptacle, or other 

 tissues surrounding or supporting the fertilized ovules. In cucurbits, peppers, and 

 in berries generally, it is formed from the ovary wall and placentae, in the tomato 

 mainly the placental tissue, while in the apple the fruit tissue derives from the 

 floral tube, and in the strawberry it is the receptacle. The ovule, which on fertili- 

 zation becomes the seed, is contained in the carpel. This is considered from the 

 evolutionary viewpoint to be a spore-bearing leaf folded over in two and fused 

 into a closed vessel (Bailey and Swamy, 1951). It is the carpel, with the ovule in- 

 side, which is the critical structure composing a fruit, and the succulent tissue is 

 often quite secondary. 



The growth of this tissue follows, and in good part results from, fertilization of 

 the ovule. In almost all fruits, cell division occurs rapidly in the earliest stages, but 

 comes to a stop either at the time the flowers open or shortly after fertilization has 

 occurred. In the apple it lasts for 3 to 6 weeks. The visible growth of the fruit is, 

 then, almost pure cell enlargement and in most cases the enlargement is roughly 

 isodiametric. It is usually very extensive. The cells of the apple enlarge up to 100 

 times their initial size; in watermelon they even become visible to the naked eye. 

 Large-fruited varieties may differ from small-fruited varieties in (a) prolongation of 

 the period of cell division, (b) greater cell enlargement, or (c), as in the pumpkins 

 and gourds, both (Sinnott, 1939). It must be understood that the apples, plums, 

 berries, tomatoes and squashes of commerce have been selected and bred for maxi- 

 mum cell enlargement over a great many generations and that, as every gardener 

 knows, full size is not achieved without generous fertilization. The factors essential 

 for cell enlargement, then, are an adequate supply of water and nutrients, espe- 

 cially nitrogen (early in the season) and potassium. The nitrogen of the apple does 

 not increase proportionally to its fresh weight, but rather to its surface; this means 



Literature p. 8i6 



