FACTORS IN EMBRYOGENESIS 15 



In different instances, different factors may assume greater or less 

 importance in determining polarity. In many bryophytes and pteri- 

 dophytes, for example, the embryo is typically exoscopic, i.e. the apex 

 emerges through the neck of the archegonium ; but in some, in which a 

 suspensor — a feature which arises in relation to the genetic constitution 

 — is present, the embryo is endoscopic, i.e. directed away from the 

 archegonial neck. The embryos of flowering plants may also be 

 regarded as being endoscopic. In each instance the polarity of the 

 embryo is determined by the time the first cleavage of the zygote 

 appears. There is experimental evidence that, in some species, the 

 plane of this first cleavage may be determined by pressures. In a 

 majority of angiosperms, however, since the zygote projects freely into 

 the semi-liquid contents of the embryo sac, pressure cannot be the 

 determining factor. The effects of gravity and light are known to 

 be important and as we have seen, genetical factors, whatever the 

 nature of their action may be, cannot be excluded. The direction 

 of the main gradients of nutrient substances, and in particular 

 of growth-regulating substances, may be important in determining 

 polarity. 



Auxins and Polarity. The action of growth-regulating substances, or 

 their precursors, is closely bound up with their polarised movement in the 

 plant. Auxins, for example, typically more basipetally, i.e. from the shoot 

 apex downwards to the root and only to a slight extent in the opposite 

 direction. It has been demonstrated that auxins can move with great 

 rapidity from one part of the plant to other parts. This movement is 

 not, however, equally rapid in all directions, i.e. from the point of 

 origin, or, in experiments, from the point of application. The move- 

 ment of auxins, which is considerably more rapid than diffusion, may 

 take place through parenchyma, phloem, or, more generally, through 

 vascular tissue. It is a reasonable hypothesis that the polarity of the 

 shoot, or of the organ involved, in some way affects the movement of 

 growth-regulating substances. The suggestion of Czaja (1935) that 

 polarity is due to the movement of auxins and not the cause of it, has 

 not met with general acceptance. The evidence indicates that the polar 

 transmission of stimuli is due to the polar transport of auxins. Accord- 

 ing to Went and Thimann (1937), polarity in auxin transport is probably 

 determined by some inherent property of the living cells : it is therefore 

 difficult to influence by changing the external environment. The 

 nature of this property is not known. Van der Weij (1934) observed 

 that the direction of transport is quite independent of the gradient of 

 auxin concentration. Went and White (1939) have shown that the 

 polar movement of indoleacetic acid is much more pronounced than 

 earher workers had realised, only high concentrations being transported 



