HAUSTORIAL ORGANS OF EMBRYOS 243 



meability of this cuticle depends upon aberrant chemical composition, 

 or upon special structural peculiarities such as the presence of ultra- 

 microscopic pores is a question which cannot be answered in the 

 present state of our knowledge. 



In conclusion, it is necessary to consider whether the living proto- 

 plasts of water-absorbing trichomes take any direct part in the process 

 of absorption by acting, as it were, like so many tiny pumps or 

 whether, on the other hand, the entrance of water is merely a physical 

 consequence of the osmotic suction produced in the mesophyll cells 

 by transpiration. In the latter event the living elements of the 

 hairs would merely serve as readily permeable passage-cells ; although 

 this hypothesis cannot be regarded as an impossible one, and in fact 

 does in all likelihood apply to the less specialised types of absorbing 

 hairs, it seems more probable that, in other cases, those elements of the 

 hairs which are distinguished by the presence of well-developed proto- 

 plasts (and also the passage-cells of aerial roots) do actually take an 

 active part in absorption, by providing the requisite energy for " pump- 

 ing " water into the mesophyll. The question is, however, really a 

 physiological one, and can therefore only be settled by experiment. 



III. THE ABSORPTION OF ORGANIC FOOD-MATERIALS. 



A. THE ABSORBING SYSTEM OF EMBRYOS AND SEEDLINGS. 



Every young sporophyte may be regarded as a parasitic organism, 

 at any rate during the early stages of its development, when it is 

 entirely dependent upon the reserve materials stored in the maternal 

 tissues. This period of parasitic existence begins with the first forma- 

 tion of the embryo, and continues until the end of germination. The 

 arrangements which are connected with the absorption of plastic 

 materials, in such cases, may be grouped under two heads. The first 

 of these comprises the haustorial organs that transfer plastic materials 

 to the embryo, while it is developing during the ripening of the fruit, 

 and is therefore still in connection with the mother-plant. The second 

 class includes all the structures which assist the young seedling to 

 absorb reserve materials, when the seed begins to germinate, after it 

 has separated from the mother-plant and has passed through a more or 

 less prolonged period of rest in the ground. 



Haustorial organs of the first type may be developed from struc- 

 tures of the most varied morphological value, such as the embryo-sac, 

 the endosperm, the suspensor, the antipodal cells (in certain Compositae 

 and Rubiaceae), one of the synergidae {Calendula), or even the pollen- 

 tube (Cucurbita, according to Longo). From among these various 



