THE ANGIOSPERMAE 1457 



of the sac where they multiply freely. Cell formation is limited to the micro- 

 pylar end. We thus see an interesting intermediate between the two main 

 types of development. 



2. The primary wall is transverse, but one or both of the daughter cells 

 divide lengthways. Examples are: Scutellaria and Verbascmn. In Calli- 

 triche, two transverse walls are formed and each of the three cells thus 

 formed divides lengthways (Fig. 1335 C). 



3. The primary wall is transverse, but one or both of the daughter cells 

 divide transversely. Examples are: Ericaceae generally, and Annonaceae. 



4. The primary wall is oblique. Example, Myosotis arvensis, where the 

 two cells formed are of unequal size. 



5. The direction of the primary wall is not constant. Examples are: many 

 Valerianaceae, Senecio and Giinnera. 



The helobial type of development begins with the formation of a trans- 

 verse wall at the first division of the endosperm nucleus (Fig. 1335 D). 

 This separates two portions of the embryo sac, a small antipodal portion 

 and a much larger portion comprising the greater part of the embryo sac. 

 In the small antipodal portion the nucleus may remain undivided, or at 

 most it divides only a few times. Occasionally as many as sixty-four nuclei 

 may be produced, i.e., there are six divisions. Wall formation may also 

 sometimes occur, so that a small mass or cushion of cells may underlie 

 the main endosperm. 



In the larger, micropylar segment of the embryo sac free nuclear multi- 

 plication takes place, as in the usual nuclear type of endosperm, followed by 

 peripheral cell formation. The growth of this tissue usually crushes the 

 antipodal development, which disorganizes and disappears. 



In the Helobiales, the first division wall is transverse, but some Bora- 

 ginaceae, e.g., Lycopsis, show a modified helobial type in which an oblique 

 wall is formed at the second division, cutting off a small lateral pouch in 

 which only a few nuclei are formed, the main endosperm developing in the 

 larger chamber of the sac. 



We have already mentioned that endosperm development may cease at 

 almost any stage, so that little or none may be visible in the ripe seed. Of 

 this suppression, there are innumerable examples, including the Orchi- 

 daceae, in which there is generally no development at all, though in a few 

 species one or two nuclear divisions take place. Even fully developed endo- 

 sperm may, however, be absorbed and obliterated by the growing embryo, 

 the food material so absorbed being usually stored in greatly enlarged coty- 

 ledons. This is characteristic of some large-seeded families such as the 

 Fagaceae, Papilionaceae and Cucurbitaceae and it is also general among 

 Compositae. In all these families the embryo finally fills the whole 

 seed. 



There are too many individual cases of peculiarity or anomaly in the 

 formation of endosperm to be dealt with, except in a specialist monograph, 

 but we may mention only the production of caeca or haustoria, w^hich are 

 prevalent in a variety of forms. Endospermal haustoria are of two kinds, 



