146 THE CAUSES OF SPECIFIC SHAPE 



to grow if in protoplasmic connexion with neighbouring cells *. The 

 laticiferous tubes of Euphorbias afford a further instance of the retention of 

 a power of apical growth in cells which are specifically differentiated 2 . 

 Certain meristems of restricted activity have possibly lost their generalized 

 embryonic characters, but it appears that the phellogen and other 

 cambiums possess general powers of development, although under existing 

 conditions their formative activity is closely restricted to certain channels. 



Plants possess great powers of reproduction, owing to the fact that 

 the meristems of the different parts retain their general embryonic character 

 and power of development. In animals, however, this power of reproduction 

 is lost owing to the high specialization, and hence it is impossible to say 

 whether a meristem of normally restricted activity might reproduce the 

 entire animal under appropriate conditions ; whether, for example, the 

 meristematic tissue which replaces a missing limb of a Crustacean might 

 be gradually induced to reproduce the entire animal. In worms this power 

 seems to be possessed to a greater or less extent by those tissues which 

 remain capable of growth. The power of development of a cell cannot, how- 

 ever, be told from its external appearance, and in fact cells may remain 

 embryonic, although they take on the appearance and even the properties 

 of differentiated somatic cells. Any cell capable of directly or indirectly 

 reproducing the entire organism must possess an entire replica of the 

 original germ-plasma or idioplasm. These general powers may be modified 

 or suppressed under normal conditions, so that the cell only exercises its 

 full powers of formative activity when the inhibitory or retarding influences 

 which normally act upon it are removed. 



Instances have already been given to show that in many cases special 

 stimuli are required to awaken a plant from a resting period, and that such 

 resting periods may be either automatically induced in the entire plant or 

 in certain organs at particular periods in the life-cycle, or they may directly 

 or indirectly result from such agencies as cold, drought, and even oxygen 

 in the case of anaerobes. Even when special chemical stimuli are required 

 to induce germination, the fact that no living or nutrient substance need 

 enter the seed or spore shows that these possess the germ-plasma in its 

 entirety and also a sufficient supply of food. Similarly the fact that an 

 ovum can only develop when fertilized does not show that a portion of the 

 germ-plasma is absent from it, for the direct development of the ovum 

 might be prevented by an automatic inhibition which the stimulus of 

 fertilization removes. This must in fact apply to all sexual cells which are 

 capable of parthenogenetic development with or without the aid of special 



1 Cf. Townsend, Jahrb. f. wiss. Bot., 1897, Bd. xxx, p. 484. 



2 The laticiferous tissue develops and grows as an independent circulatory system, but 

 nevertheless every meristem which can produce a new plant has ipso Jacto the power of producing 

 a new laticiferous cell. 



