1/8 LECTURE X. 



We have now obtained a general insight into the mode of 

 distribution of the plastic materials in the plant, as well as 

 some knowledge of the different forms in which they may 

 make their appearance. We will conclude our consideration 

 of this subject with a few general considerations, leaving the 

 discussion of the chemical details for another lecture. 



We may gather from the facts before us that the parts of 

 the plant which do not contain chlorophyll are, as it were, 

 parasitic upon those which do. Just as a plant which does 

 not contain chlorophyll must have organic substances supplied 

 to it, so must also those parts of a green plant which do not 

 contain chlorophyll. The cells in which chlorophyll is 

 present can make organic substance, both nitrogenous and 

 non-nitrogenous, from carbon dioxide, water, and salts ; those 

 in which it is not present can only decompose and recombine 

 the organic substance supplied to them from those in which 

 it is present. This relation is well marked in the develop- 

 ment of shoots from bulbs, tubers, etc.; a shoot is incapable at 

 first of constructing its own plastic materials, and draws its 

 supplies from the depository of reserve-material with which it 

 is connected ; it behaves like a parasite. On the other hand, 

 as far as continuity of tissue is concerned, a parasite is as 

 closely connected with its host, as a shoot is with the organ 

 which bears it. The relation of the embryo in this respect is 

 peculiar. In albuminous seeds, the embryo is simply im- 

 bedded in the endosperm (or perisperm) ; there is no con- 

 tinuity of tissue. When the seed germinates and the further 

 development of the embryo commences, the reserve-materials 

 in the endosperm undergo the changes with which we have 

 become acquainted, and the products are absorbed by the 

 embryo through its external surface ; in some cases a special 

 absorbent organ is present, as in Grasses (the scutellum), and 

 at a later period in the development of the embryo the 

 cotyledons frequently act as absorbent organs. 



Treub's interesting observations on the embryo of Orchids, the sus- 

 pensors of which grow out of the ovule, in certain species, and attach 

 themselves to the placenta from which they absorb nourishment, seem to 

 suggest that possibly the suspensor may have an absorbent function in 

 all cases. 



