ON FORMATIVE INDUCTION AND PECULIARITIES OF CELLS 145 



Mechanical pressure is not, however, responsible for the non-formation of 

 leaves on stem-tendrils, or for the differentiation of leaf-primordia into leaves, 

 petals, or stamens according to the determining factors due to their position. It 

 is also due to internal processes of automatic regulation that a flowering plant 

 ultimately produces floral leaves with a different phyllotaxis to that of leafy 

 branches. When a flattened cactus stem becomes triangular the phyllotaxis 

 changes from \ to i 1 , and this is to a certain extent comparable with the fact that 

 in roots the number of vascular bundles determines the number of rows of 

 secondary roots. It is therefore evident that a regular arrangement of the lateral 

 appendages is easily possible, even when they never come into contact and hence 

 do not exert mutual pressure. The fact that leaf-primordia may appear where 

 there is most room for them, or where the pressure is least, is not necessarily 

 the result of a direct mechanical action, but may be due to internal processes of 

 automatic regulation, possibly aided by the stimulatory action of differences 

 of pressure. 



SECTION 42. Formative Induction (continued). 



The varied metamorphoses which the equipotential segment-cells 

 undergo after the divisions of the meristem have ceased may involve the 

 gradual or almost immediate loss of the embryonic character. If, however, 

 divisions occur in cells which have already undergone slight differentiation, 

 the products may be dissimilar from the outset, as is, for example, the case 

 when a cell of Spirogyra separates a non-nucleated segment from itself 2 . 

 In other cases dissimilarity may exist between the two segments, although 

 no external sign of it is immediately perceptible. It is then naturally 

 difficult to say whether the differentiation began previously to the division, 

 during it, or after it was completed. Such growing cells are as readily- 

 capable, within the limits possible to them, of responding to stimuli by 

 changes of shape as are purely embryonic undifferentiated cells, whose 

 general powers of development are always held in check and regulated by 

 the conditions existing at the time. 



A pollen-grain with its pollen-tube affords a good instance of a 

 specially differentiated cell which retains a power of active growth 3 . In 

 living tissues, moreover, a non-nucleated mass of protoplasm would be able 



1 Vochting, Jahrb. f. wiss. Eot., 1894, Bd. xxvi, p. 484. Cf. also Kny, Ber. d. Bot. Ges., 

 1898, p. 60; Weise, ibid., 1899, p. 343; Correns, Festschrift fur Schwendener, 1899, p. 395; 

 Church, The Relation of Phyllotaxis to Mechanical Laws, Pts. I-IV, 1901-2. 



a Gerassimoff, Ueber die kernlosen Zellen bei einigen Conjugaten, 1896; Ueber ein Verfahren, 

 kernlose Zellen zu erhalten, 1896 (jepr. from Bull. d. 1. Soc. Imp. d. Naturalist.es d. Moscou) ; 

 Townsend, Jahrb. f. wiss. Bot., 1897, Bd. XXX, p. 484. 



8 Pollen-tubes may be grown to a considerable though always limited length. It remains, 

 however, possible that either the pollen-tube might be able to reproduce its kind, or that the 

 pollen-grain possesses general embryonic properties and mi^ht be caused to reproduce an entire 

 plant. 



PFEFFER. II L 



